viewer: migrate to appstream

Include appstream-xml.m4 in m4 subdirectory.
tests: avoid a crash in camera-test when chunk are not used
doc: gtk-doc fixes
camera: add GigEVision specific API
realtime: test program

Depends on dbus for now.
tests: port realtime test to gdbus
tests: RTTimeUSecMax is RTTimeNSecMax on ubuntu

WTF ?
realtime: new API for making stream thread realtime
realtime: fix return value of make_thread_realtime

If sched_setscheduler succeed, return TRUE, no FALSE.
realtime: API documentation
viewer: make stream thread high priority