Add a cmd+a keyboard shortcut to select all tracks

I frequently work across coordinated work between the parent process, content processes, and utility processes. Here I like to use the marker view and combine markers together. It takes a bit of time to cmd click or shift click the tracks to select them all. It would be better ergonomics to allow for cmd+a (ctrl+a) for selecting all of the tracks.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-1395)

Start in the marker view and trace the existing track-selection handling, including how cmd-click and shift-click are processed. Add keyboard selection behavior for cmd+a and ctrl+a so all tracks become selected, then verify the shortcut in the marker view and add or update coverage if an existing selection test entry point is found.

Written by the indexing model from the issue text.