timeline: Selection cleanup
timeline: Remove Selection.last_single_obj as it's not used
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: 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
timeline: Allow zooming in when the best fit button is pressed
dogtail tests: Fix effects tests
dogtail tests: Rename variable to clarify what it represents
dogtail tests: Reuse center method