Added loading of video node factories from plugins

Added loading of video node factories from plugins

This change will allow us to add new video node factories that
can be loaded dynamically at runtime.

The previous video node factories, I420 and RGB have been kept
as static parts of the Qt Multimedia imports, but can be moved
to plugins at a later date.

For plugins to be able to find and use QSGVideoNode, the class
has to be exported.

Change-Id: Idbead9a8ad33619cebe90fcec92eb29cf52ae9bd
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>