Fix/Improve PianoRoll Knife Tool UI

Currently the knife tool in the piano roll is hidden behind the tool menu, when it probably should reside along with the other modes buttons, such as draw/erase/select/pitch bending.

Also, it would probably be good to add a TextFloat which appears when the user enters the knife mode giving some simple tips/instructions, particularly that you can use Shift to automatically remove the cut ends of the notes.

I am mostly just making this issue as a reminder for me to come back and fix these things, but if anyone wants to tackle these, feel free!