Introducing Windows Runtime backend

This lays the foundation for using supported APIs in Media Foundation
for the Windows Runtime family of devies. Due to the narrowing of
supported MF classes, as well as classes that are only available on
Windows 8 and newer, this backend is built as a new plugin rather than
a modification to the existing wmf plugin.

[ChangeLog] Added a new backend plugin, winrt, to support multimedia
within Windows Runtime OSes.

Change-Id: If64ac417127b7400baa5e56fe02b65f909a4dece
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>