GLib watches and timeouts require GLib mainloop iterations. If the GLib
mainloop is not running, then GLib watches and timeouts won't trigger.
Back in the GTK+ days, then GLib mainloop was running on all systems.
Since the Qt transition, GLib mainloop only runs on Linux when Qt does
support it and environment variable QT_NO_GLIB=1 is not set.
Start polling GLib mainloop in separate thread if Qt is not running GLib
mainloop. Note that only the polling is handled in separate thread, the
dispatch and thus all user callbacks execute in the main thread.
Running GLib mainloop when needed enables full GLib functionality on all
platforms and thus allows us to simplify our code by using GLib platform
specific code.
Tomasz Moń