Qt integration (WIP)

* Added main loop abstraction layer which can use either GLib or Qt as a backend. (thanks John!)
Implement mainloop_running(), QueuedFunc::queued() and TimedFunc::running() and make use of them
Migrate vis-runner to new mainloop
Use simpler version of the main loop abstraction layer
Revert changes in libaudgui
Migrate eventqueue to new main loop
Merge branch 'master' into qt-integration
Fix a compiler warning.
Move Qt detection to configure.ac as it is different between core and plugins.
Merge branch 'qt-integration'