Piano roll fixes

[swesterfeld]

This branch contains some minor fixes/improvements for the piano roll.

• it contains a key binding (shift+up/down) to move notes one octave up/down (which is a pretty common operation)
• it draws selected notes over unselected notes, which is more intuitive
• it fixes the odd situation that if you had an unselected note which overlapped a selected note, your tool (sometimes) affected the unselected note, which is note intuitive, so now selected notes get a higher tool priority