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.

this commit deprecates qtwebkit usage.

used from both entrypoints for linux and mac apps.

some juggling to make systray (qt5 for now) and browser (pywebview,
native) start and stop coordinatedly.

I will explore a more lightweight systray for coming releases.