VideoMultiCameraCaptureDemo: Add more test code for sync'ed stop of capture.

A keystroke will use the 'StopAtFramecount' setting to schedule logical stop
of capture at current framecount, but then keep the frame fetching+display
loop running until a second keystroke really stops capture.

This allows to test that frame-fetching with effectively stopped background
thread still works, as long as capture is not really stopped via 'StopCapture'.
The latter will release the whole GStreamer pipeline, ergo the ptbvideosink,
ergo all queued frames.

-> Works.