Fix/Improve PianoRoll Knife Tool UI

[regulus79]

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!