Add support for switching between left/right contour priorities

[sonicpp]

I use this already in practice for some time. When contour is active, new button for selecting priority (left or right) will appear (so it does not take up space when it's not active). Left/right priority will be set depending on this new button status - if it is checked or not (might not be ideal but works good).

Tested on AMD64 and Aarch64 (Rpi4)