New MediaPlayer image with timeline

Enable playing and seeking into a timeline on a media player that loaded an image. Timeline sets a duration (end) and is saved/loaded. Add a gstreamer imagefreeze element in the pipeline to simulate a playable stream. Distinction must be made between 'isImage' (what was loaded) and 'singleFrame' (what is in the pipeline). GUI is added and customized with menu and dialog.

src/SourceControlWindow.h

@@ -48,6 +48,7 @@ class SourceControlWindow : public WorkspaceWindow
     // Render a single media player
     MediaPlayer *mediaplayer_active_;
     bool mediaplayer_edit_fading_;
+    bool mediaplayer_set_duration_;
     bool mediaplayer_edit_pipeline_;
     bool mediaplayer_mode_;
     bool mediaplayer_slider_pressed_;