WIP mark the point where we lost the draw() callback during GTK3-transition

actually we should make our timeline a real custom widget,
and do it according to the letter. I.e. really implement
all those callbacks which are recommended, but no other
callbacks.

This has the additional benefit of being able to retrieve
the drawing style in the official way, and define our own
CSS classes, which can be styled by the user in a systematic way.

src/gui/widgets/timeline/timeline-body.hpp

@@ -102,6 +102,7 @@ protected:
   
   /**
    * An event handler for when the window must be redrawn.
+   * @todo looks like this was the main drawing hook, and happens to be discontinued in GTK3.   ///////////////////TICKET #937 : custom timeline widget draw function not invoked anymore
    */
   bool on_expose_event(GdkEventExpose* event);