[feat] Let's do "fugue machine" #2: Display all tracks at once

Second of some ideas to give us a UX a little more like FM.

As $subject, e.g. pressing and holding key 2 causes the grid to use all eight rows to display all four tracks as they appear on their individual pages.

Alternately (or maybe stretch goal? ;p), assign a grid key toggle and make this a new page, i.e. sequences are playable/editable the same as on their own individual pages.