- move snap folder to root folder, to allow automated builds
- install only polkit file outside of snap
- change path of bitmask-root accordingly in bitmask
- fix check for polkit file inside snap
- change and document the algorithm for picking bitmask-root in linux
- add LD_LIBRARY_PATH as an environment entry for bitmask-systray

for some usages, specially with autostart enabled, user might want to
launch only the systray.

this commit implements a simple ``--nowindow`` switch that just avoids
showing the main window for now. in the future, we can have a different
entrypoint that just launches bitmaskd and a minimal systray widget.

I'm not documenting this feature properly since I think this is still
missing some functionality: the ability to switch on and off the vpn,
and the ability to pass the --autostart as a flag to the bitmask
entrypoint.