timeline: Make the timeline 2D
mainwindow: Use existing field instead of passing it as a parameter
Select the proper context tab when a clip is selected
tests: Add unittests for Selected and Selection
medialibrary: Add half space unit at the left of the search field
Focus the timeline by default

This fixes a bug: when the app is started and a project is loaded, then
the media library search field is clicked and "s" is typed it has effect
on the timeline. This happens because the timeline deactivates they
key-actions only when it loses focus.
titleeditor: Make the notebook page switch handler more robust
mainwindow: Show by default the title editor tab
mainwindow: Short and consistent names for the context tabs
clipproperties: Fix effect selection in the clip properties tab

Should have been part of commit c89e31eb.

Fixes bug https://bugzilla.gnome.org/show_bug.cgi?id=720697
elements: Fix video clips keyframes and lines vertical position

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=711168