Add UndoButton to Shotcut.Controls.

And this fixes a missing icon on Linux when using System theme where the
system theme does not have an edit-undo action icon.
Add timeline dock with multitrack model (WIP).

This is in a very basic beginning, read-only state that basically proves
the concept of using QML against a hierarchical model over MLT.
Use Mlt::Properties time_to_frames() and frames_to_time().
Improve scrolling in timeline.
Add timeline zoom slider and auto-scroll.
Add video thumbnails to timeline.
Move timeline scale slider to bottom.
Only render timeline playhead once.
Add audio waveforms to timeline.
Fix appearance of blank item on timeline.
Add saving changes in the track header.

Track name, mute, and hide toggles.
Refresh immediately when toggle hide track.
Add single clip selection.
Add basic drag-n-drop interactions to timeline.

This does not yet affect the model.
Fixes for Qt 5.2.
Change waveform (audio levels) data to use entire clip.

This way, it will be cheaper to change in and out points by reusing this
data and simply rendering different portions of it.
Fix scrubbing timeline near start.
Connect trimming on timeline to model.
Add moving clips around on same track.

Does not yet support moving between tracks or dropping clip onto another
clip to split or create transition.
Add snapping to clip on track of timeline.