Handle gstreamer signals on main QGC thread to avoid deadlock; Remove video sink from pipeline if pipeline failed to start

Makes more sense for it to be in VideoStreamer as it takes care of managing
the video streams.

Fix matroskamux stream start (workaround for https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/issues/449)

Use parsebin for automatic stream demuxing and parsing; Automatically insert rtpjitterbuffer for RTP streams to avoid video stuttering

Switch to decodebin and use decoder with highest rank; Use GL context provided by qmlglsink; Fix JNI code for AndroidMedia initialization

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

Allow developers (or advanced users) to override UI settings - quite useful if you set your debug settings in QtCreator and do not depends on QGC settings state (they are reset to default sometimes, for example)

Every single line of this function is commented out, and the
documentation of GStreamer says that it's uneeded.

Move to 64 bit, MSVC 2017