Drop unused return value
Move glib main loop thread state into a separate struct
Ensure all SPICE NDK processing runs on glib main loop thread
Use single glib main loop for all sessions

There's no way to schedule callbacks on specific main loops, so the
one-loop-per-session model is broken.
Run VMNetX at the root of exactly one task

Drop documentLaunchMode, since it only works on Lollipop.  We will need
to provide an in-app session switcher.

Ensure we are always launched into a separate task, rather than running
in the web browser's task.  If another session is already running, ask
the user for permission to terminate it.