qa: Add a GstQaReporter interface that objects needing reporting can implement

qa: Add a GstQaReporter interface that objects needing reporting can implement

Various type of object should be able to do some reporting, so we have
to make sure all the code to do that is in one place. Creating an interface
makes it simple to share information and it avoid to have a baseclass for
something that is not actually important enough to create a baseclass.

Conflicts:
	gst/qa/gst-qa-pad-monitor.c