Move the bar while packages are downloading

The progress entry sat at "0 von 123" for the whole download and only
started counting once pacman began unpacking - which on a domestic line is
most of the run spent looking like nothing was happening. What was being
watched for was the transaction, and the transaction had not started yet.

Downloading is now a step of its own, worth 30 of the bar against the
transaction's 40, counted the same way: one line per package out of
pacman's "foo-1.2-1-x86_64 downloading...". The database sync just before
it prints the identical shape with the suffix that would give it away
already stripped, so the tally starts only after the ":: Retrieving
packages..." header. Packages already in the cache never announce
themselves, so the step regularly ends short of its total and hands the
rest of its share over when unpacking begins.

The headline follows: "Updates werden heruntergeladen", then
"Systempakete werden aktualisiert".

po/cachy-auto-update.pot

@@ -234,6 +234,9 @@ msgstr ""
 #. looking for it - it appears next to whatever they were doing - so each one
 #. says outright that this is an update running, rather than naming the kind of
 #. package on its own.
+msgid "Downloading updates"
+msgstr ""
+
 msgid "Updating system packages"
 msgstr ""