anykey: Use GLib I/O channel watch

Watch for input on a GLib I/O channel using the standard GLib main
loop API instead of injecting an FD event source into the sigrok
session loop.

Also run the session in a main loop created by sigrok-cli instead
of using sr_session_run(). This is mainly for demonstration; the
GLib I/O watch change would work even with sr_session_run() since
it is using the same main context in this case.