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

autostart-dropins/org.gnome.SettingsDaemon.XSettings.desktop

@@ -0,0 +1,2 @@
+[Desktop Entry]
+OnlyShowIn=GNOME;X-QUBES