winrt: Add camera service

This adds a basic camera service with viewfinder (video renderer based),
still image capture, and device selection support.

Runtime apps must set the "webcam" and "microphone" device capabilities
in order to access the hardware. This can be done by adding the following
to the .pro file:
    WINRT_MANIFEST.capabilites_device += webcam microphone

[ChangeLog] Enabled basic camera support in the winrt backend.

Change-Id: If4f963ef645d93c757ae23aec9a9c8aae122324f
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>