Separate "SimpleTimeline" actions into an interface
@bilboed
Submitted by Edward Hervey Assigned to Edward Hervey @bilboed
Description
pitivi.timeline.TimelineComposition is getting huge, and most of that is due to the SimpleTimeline actions.
All those actions should be moved to a separate interface.
Imported from https://bugzilla.gnome.org/show_bug.cgi?id=396806