feat: turn off KDE's middle-click paste

Inside a Chromium application the flag settles this on its own - with
autoscroll on, Blink stops pasting the primary selection. Everywhere else
on the desktop middle click goes on pasting, which is the thing this whole
program exists to get away from.

KWin has a switch for it, on Wayland, where the paste is a protocol the
compositor either offers or does not: EnablePrimarySelection in kwinrc.
Written with kwriteconfig, the way System Settings would, because kwinrc
has a cascade behind it. What the key said before goes in the ledger, so
disable puts it back - and a key that already said false is left alone and
not recorded, so a paste the user switched off themselves is never
switched back on.

X11 has no such switch and nothing is written there.

README.md

@@ -18,6 +18,7 @@ Just run `middleclick-autoscroll`. This will open the configuration TUI that loo
   Not identified                 1 - see the applications list
   Steam                          ON
   New applications               ON
+  Middle-click paste             off
   Last applied                   2 minutes ago
 
   Applications pick this up the next time they are started.