elements: Fix RoundedRectangle.get_border_color parameters
timeline: Always set self.current_group in TimelineStage._dragEndCb
timeline: Selection cleanup
timeline: Remove Selection.last_single_obj as it's not used
project: Avoid setting the selection field for a GESTimeline object
timeline: Remove unused addClip method
timeline: Raise a proper exception if no element is found
timeline: Remove unused imports
timeline: Use proper value in debug statement
timeline: Avoid zooming in when setting the best zoom ratio
widgets: Fix the zoom slider not showing the initial zoom value