Initializes GStreamer library and tweaks some of its defaults to our liking.
This also allows usage of GstClapper API alone without importing GStreamer on the implementation side.
It changes preferred hardware acceleration order (depending on plugins versions) into:
v4l2sl -> nv -> va -> vaapi -> v4l2
The "clapper_src" directory name was unusual. This was done to make it work as a widget for other apps. Now that this functionality got removed it can be named simply "src" as recommended by guidelines.
