Trace viewer: move mouse mode selector into top controls bar.
- shift, control temporary modes
- move buttons to left controls?
Currently, the mouse-mode-selector is a complex dom-module with many
responsibilities. It is used in a few places: timeline-track-view, quad-stack-view,
This CL removes the mouse-mode-selector from timeline-track-view, and adds 4
mouse-mode-icons to timeline-view, which control timeline-track-view.mouseMode.