cot: dynamic theme chooser (1/2)

This works, but there's still some bugs, namely:

1. A back swipe causes recovery to go all the way back to the main
menu.
2. Nothing is done with the selection (yet)
cot: theme engine mods for multitheme
cot: complete theme engine

This commit depends on the settings implementation
which is not finished yet. This commit builds fine,
but you are locked into the default theme for now.