Introduce EME support using Discretix DRM Client

This branch synchronises EncryptedMedia extension code with upstream and implement a CDMSession provider in the GStreamer media player backend. The player will fire the webkitneedkey event providing DRM initialization data and will handle license acquisition dialog together with the JS code. GStreamer pipeline will wait for a valid license in the mean time. This wait can be terminated by a valid license being added or by the player changing state (loading another URI).