Arrow keys should navigate by selection

[jminor]

Using the arrow keys, the user should be able to move the selection left/right to adjacent items within a track, or up/down to items in the previous or next track. This navigation should work for gaps, and transitions also.

If/when nested compositions are supported, there should be a way to navigate in/out of nested compositions (maybe Enter and Escape?) See #72

Question: If a marker is selected, should the arrow keys should step from one marker to the next, or is this covered by the Markers tab in the Inspector?