Make sure we repaint after user switching on X11

This makes sure we're redrawing the stage after having user switched (or VT switched for that matter). This fixes an issue where the screen would remain black after having switched back to an active X11 session.

Note that this adds a default, but disable:able, dependency on having a logind provider available even when building without the native backend.

Whether this should target 3.32 or 3.34 is an open question. It's purely a bug fix, and I have made the changes so far to minimize the possibility of breakage by not porting over MetaLauncher functionality to the logind integration in the generic backend layer, but so is my plan for 3.34.

An alternative for making this more suitable for 3.32 if the logind provider dependency is deemed unsuitable, would be to make logind provider dependency automagic on the 3.32 branch, but leave as is (default on, possible to turn off) in this MR on master.