Major milestone reached: note velocity editing

Polymeter reached a major milestone today: note velocity editing! The velocity pane is an optional addition to the step view. It's resizable via a splitter bar, and stays synchronized with the step view's scrolling and zoom. Multiple tracks can be edited at once, in which case their note velocities are merged. Various refinements are still in progress, e.g. a tool tip showing the current velocity, and fancy editing behaviors such as clamping velocities to a line segment or periodic function. The velocity editing pane can be reused to edit other values besides notes, e.g. controller values, and is thus a big step towards the next milestone, which is controller tracks. However controller tracks will require significant changes to the sequencer core, particularly for linear interpolation.