Add timeline ruler into main keyboard navigation hierarchy

### Description

The timeline ruler needs to be added into the main keyboard navigation hierarchy following the "Add track" button and preceding the first track.

Pressing shift + f10 with the timeline ruler in focus will summon the right-click context menu.

With keyboard focus on the timeline ruler, the User should be able to use the left/right arrow key shortcuts to move the playhead left and right. All modifiers should also be able to applied when performing these left/right arrow key moves.