Close#10867.
It means that settings couldn't be saved. The issue appeared
since upgrade to nixos 15.09 and fall as part of #4415.
Tested on nixos.
- No longer has a stderr when running the application.
- Settings are effectively saved.
- forgotten mousepad update, including some wrapping magic
- dealing with panel plugins (either fix or mark as broken)
CC maintainer @AndersonTorres.
- remove some libxfcegui4 occurrences, as it's being phased out
- minor stuff
