Adds an interface method to the FrameCapture class to get the device
display info (device address and device display id) for each display of
the graphics device that is captured.

Also adds functions to the API implementing this functionality for X11
in variants with and without DRM (the non-DRM version is rather limited
and may not work for more complex setups) as well as some helper
functions to make it easier for plugins to implement this and avoid code
duplication.

Implements the new interface method for the two built-in plugins
(mjpeg-fallback and gst-plugin).

Signed-off-by: Lukáš Hrázký <lhrazky@redhat.com>
Acked-by: Jonathon Jongsma <jjongsma@redhat.com>