Add media DRM API

Currently, protected (DRM-encrypted) media content cannot be played with the Qt media player. We need an API to manage DRM content and the corresponding implementations inside each plugin.

The API should allow to:

• Query whether the media source contains protected content
• Rights management (register / unregister, acquire, query status)

As for actual implementation, we can support DRM in the following plugins:

• Windows (PlayReady, Windows Media DRM): https://msdn.microsoft.com/en-us/library/windows/desktop/ms700168(v=vs.85).aspx
• OSX / iOS: (FairPlay): https://developer.apple.com/streaming/fps/
• Android: https://developer.android.com/reference/android/drm/package-summary.html
• GStreamer: not sure what should be done there yet, GStreamer doesn't support DRM content out-of-the-box. Some third-party plugins (like Fluendo codec pack) offer such features. Should we have our own GStreamer DRM decoder element and support directly the main DRM systems (Widevine, PlayReady, FairPlay, Adobe Access, Marlin and others)?