Kill off all subprocess when exiting app.
make both sets of buttons similar heights
Implement proof-of-concept font zooming for output pane.
Provide 'auto pushing/popping' versions of `with-*` fns via blocks:

Instead of directly modifying the thread local with the new value, if
you pass a block to one of the `with-*` fns, the thread local will just
be modified for the duration of the block, and then returned to its 
previous value once the block has finished.

````
with_synth "pretty_bell"
play 30 #=> synth is pretty_bell"
with_synth "fm"
play 30 #=> synth is fm
with_synth "mod_pulse" do
  play 30 #=> synth is mod_pulse
end
play 30 #=> synth is back to fm
````
Teach autocomplete about `with-*` functions