Further refine workaround for SKC key press handing

Only consider keyboard modifiers that are in GTK's default mod mask
when deciding to engage/disengage the SKC hack in MainWindow, so things
like NumLock being on (GDK_MOD2_MASK) don't disengage the hack.

Fixes #77
5 jobs for wip/730495-fix-inline-plain-text-paste-0.12 in 6 minutes and 39 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Build
passed #102675
debian

00:04:19

passed #102677
deb-package

00:04:38

passed #102674
fedora

00:04:49

passed #102678
flatpak-package

00:06:34

passed #102676
ubuntu

00:04:14