Summary:
this is largely a partial sync from the kf5 launcher used for the kf5
content snap.

launcher:
- make sure all lib paths are in ld_library
- make sure GL drivers may be found
- make sure nvidia host-mounted drivers may be found
- rejigger font handling to sit on top of the host fonts instead of
  shipping ones inside the snap
- XDG variables are no longer separated by version (the individual HOMEs
  are already by version e.g. ~/snap/krita/{1,2,3}) so the additional
  versioning only serves to lose all settings on updates
- set more XDG variables to ensure they are sound
- force qtchooser to use suitable paths directed at the snap
- do not hardcode any SESSION variables or platformtheme. the variables
  are forwarded into the snap, and Qt will figure out what to do with the
  environment on its own
- do not attempt to start KIO slaves via klauncher but instead qprocess
  fork them (ensures a) not breaking out b) they are in...