Accessibility: Add support for dynamic text font sizes

[benediktewallace]

Some hit areas are also deemed "too small for user to interact". An example being the instrument chooser in the new performance view

[cpmpercussion]

Hm, not sure how to manage that. I may have just made it worse by adding another button in that horizontal stack view (see latest commits).

It must be close to acceptable - might just be a matter of a few pixels and can be solved.

I'm considering whether to move that dropdown button - there's quite a bit of space above the jam square (at least on 4.7in devices), might be able to have a more sensible arrangement.