New SourcePlayer

Work in progress; Sources now have play/pause and associated play functions. Media player can play all playable sources, and adapts to control a media player when possible. Selection of play groups (to finalize)

StreamSource.h

@@ -30,6 +30,11 @@ public:
     // implementation of source API
     void update (float dt) override;
     void setActive (bool on) override;
+    bool playing () const override;
+    void play (bool) override;
+    bool playable () const  override;
+    void replay () override;
+    guint64 playtime () const override;
     bool failed() const override;
     uint texture() const override;