Put a progress bar on the desktop, and repair the notifications around it

An unattended run takes twenty minutes and said nothing at all while it worked.
The notification area now carries a live entry for the duration - which step is
running, which package is being unpacked, item count, percentage. It is a job
in the sense of org.kde.JobViewServer, the same mechanism Dolphin uses while
copying files, rather than a notification, which is what makes it a real bar
instead of a line of text.

That mechanism needs a process of its own. The desktop ties a job to the D-Bus
connection that requested it and withdraws it the moment that connection
closes, so gdbus, busctl and dbus-send cannot drive one at all - every
invocation is a fresh connection that closes again immediately. A small helper
therefore runs inside each graphical session for the length of the update,
holding the connection open and taking instructions on stdin. It needs
python-gobject; without it there is no bar and nothing else changes. Position
within the repository step is read from pacman's own "(120/260) upgrading foo"
lines. Its other (n/m) sequences are ignored on purpose: checking keys, package
integrity and loading files each count up to the same total, and following them
would run the bar to the end three times before the first package was unpacked.

The "system updated" notification never arrived, which is what prompted looking
at any of this. Tagged notifications were posted with --replace-id naming the
start message, and Plasma silently drops a Notify() whose replaces_id points at
an expired notification: no bubble, no error, and the id it hands back is the
dead one it just ignored. The start bubble times out in seconds and a run lasts
minutes, so the result fell into exactly that hole every single time. The
previous message is now withdrawn and a fresh one posted.

How long a message stays is set per message rather than left to the daemon.
Anything reporting that the machine still needs a person - a failed update, a
locked package database, packages that had to be held back - waits until it is
dismissed. Everything else times out on its own, a successful update included;
nothing should have to be clicked away for having gone right. Daemons do keep
critical-urgency messages up and the specification asks them to, but that is a
should, it says nothing about the normal-urgency messages here that still need
somebody to act, and urgency separately governs sound and do-not-disturb.

"Restart recommended" is gone, and NotifyReboot with it. The running kernel
loses its module tree the moment pacman unpacks the new one, so the notice
fired while the run was still building AUR packages and pulling Flatpaks, where
it reads as an invitation to restart in the middle of an update. The state is
still recorded and cachy-auto-update status still reports it. The option went
rather than only its default, because an installed system keeps its own
configuration file and would have carried on notifying regardless.

res/applications/cachy-auto-update.desktop

@@ -0,0 +1,14 @@
+[Desktop Entry]
+# Not here to put an entry in the application menu - hence NoDisplay. This is
+# how the desktop learns what to call the update while it runs: the progress
+# bar in the notification area is labelled from the desktop entry named in the
+# job request, and without one the job is filed under whatever the helper
+# process happens to be called.
+Type=Application
+Name=CachyOS Auto-Update
+Comment=Unattended background updates
+Exec=cachy-auto-update
+Icon=system-software-update
+Terminal=true
+Categories=System;PackageManager;
+NoDisplay=true

res/config/cachy-auto-update.conf

@@ -94,11 +94,21 @@ RemoveOrphans=no
 # ---------------------------------------------------------------------------
 # Notification detail
 # ---------------------------------------------------------------------------
+#
+# How long a message stays on screen is not configurable, because it follows
+# from what the message is for: anything reporting that the machine still needs
+# a person - a failed update, a paused package queue, a package that had to be
+# skipped - waits until it is dismissed, since a message like that is only ever
+# useful if it is still there when somebody comes back. Everything else, an
+# update that simply worked included, times out on its own.
 
 # Say that an update has started. Worth keeping on: while one runs, a shutdown
 # request is refused and the desktop answers with an untranslated polkit
-# password prompt that never mentions updates. This message is replaced by the
-# result when the run finishes, so it costs one bubble, not two.
+# password prompt that never mentions updates. The message is withdrawn again
+# when the result arrives, so it costs one bubble, not two.
+#
+# Separate from the progress bar in the notification area, which is not
+# optional and appears whenever the desktop supports one.
 NotifyOnStart=yes
 
 # Say something after a successful update. Nothing is ever shown when there
@@ -108,6 +118,9 @@ NotifyOnSuccess=yes
 # Report failures.
 NotifyOnError=yes
 
-# Point out that a kernel update needs a restart. The machine is never
-# restarted automatically.
-NotifyReboot=yes
+# A kernel update that needs a restart is not announced by a notification. The
+# running kernel loses its module tree as soon as pacman unpacks the new one,
+# so that would fire while the run is still working through AUR packages and
+# Flatpaks, and reads as an invitation to restart in the middle of it.
+# `cachy-auto-update status` reports it instead. The machine is never restarted
+# automatically either way.