Reworked synchronous video recording stop

Reworked synchronous video recording stop

The issue with QtCamGStreamerMessageListener::waitForMessage() is that
gst_bus_timed_pop_filtered() will drop the messages not matching our type. This
means our handlers might not process such messages.

What we do now is pass a custom DoneHandler subclass in QtCamVideoMode which will
wait on a QWaitCondition for video-done to be received.

Since video-done is sent from the streaming thread and DoneHandler is installed
as a sync handler, we are sure to block the UI thread until video gets saved.