elements: Remove unused get_border_color method
timeline: Always set self.current_group in TimelineStage._dragEndCb
timeline: Selection cleanup
project: Avoid setting the selection field for a GESTimeline object

We set and use TimelineStage().selection = Selection().
The GESTimeline().selection = Selection() field removed in this commit
was simply not used anywhere.
timeline: Raise a proper exception if no element is found
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
dogtail tests: Rename variable to clarify what it represents
dogtail tests: Reuse center method
medialibrary: Remove unused _trackElementAddedCb method
timeline: Raise an exception when the Timeline widget has no GES Timeline
timeline: Insert the clip at the end of the longest layer
timeline: Rename caps-locked local variable
timeline: Request at least the height of a pair of layers for the timeline
widgets: Specify parameter names when making pack_start calls
Remove unused receiver class
timeline: Remove unused SELECT_BETWEEN constant
Fix source code file headers
Remove unuseful instance module

This code never worked.