Enable gnome settings daemon xsettings plugin
When one use scaling set by gnome tools (gsettings or
gnome-tweak-tool), gsd-xsettings must be running to apply the change
also to other applications.
This include auto scaling on HiDPI screens.
This commit fixes non-uniform behaviour on different VM types.
QubesOS/qubes-issues#3108
Marek Marczykowski-Górecki