General cleanup

Hi,

I did two things:
 1. **UI**: in help dialog I changed all actions (e.g., "add directory", "quit") to lowercase. Some of them needed to be in uppercase in previous versions, while the menu was on the top of the screen, but now uppercase is redundant since you made the help text much cleaner
 2. **code**: the ten subroutines `b1_switch`, `b2_switch` all had been doing just the same thing. I folded them all into one subroutine `band_switch` requiring one argument: 0..9 number indicating the band number.

I would like to hear your opinion :)