winrt: Add media playback service

This introduces a MediaPlayer service based on the MFEngineEx API. Only
scene graph video rendering is supported at this time. The video renderer
control is abstracted in order to provide a base for future video nodes
which do not source their content from the MF engine.

[ChangeLog] Media player support was added to the winrt backend.

Change-Id: I8155a1030466ea352fad0a87d1ae97a88983760c
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>