video_input.cpp

This patch moves the video source parsing logic from the VideoInput
class to the VideoInputSelector. Thus the VideoInput class is simpler
and only deals with libav input.

The VideoInputSelector class now takes as its switchInput() parameter
(and thus the constructor parameter) a media resource locator (MRL) as
commonly used by video players such as VLC.

Currently, "v4l2://" and "display://" schemes are supported.

The DBus API and documentation are updated accordingly.

In the meantime, this fixes the confusion (and thus a bug) with a camera
having a ':' char in its name, which was resulting in using the x11grab
format.

Refs: #45480