A PianoRollScoreMorph displays a score such as a MIDIScore, and will scroll through it tracking the porgress of a ScorePlayerMorph (from which it is usually spawned).

timeScale is in pixels per score tick.

Currently the ambient track (for synchronizing thumbnails, eg) is treated specially here and in the score. This should be cleaned up by adding a trackType or something like it in the score.

	drawing
	addNotes drawMeasureLinesOn: drawOn: drawStaffOn: isBlackKey: rebuildFromScore

	dropping/grabbing
	acceptDroppingMorph:event:

	editing
	appendEvent:fullDuration: copySelection cutSelection deleteSelection insertSelection insertTransposed

	event handling
	handlesMouseDown: mouseDown: mouseMove: mouseUp:

	geometry
	contractTime expandTime extent: fullBounds layoutChanged midiKeyForY: tickTimeAtCursor timeForX: xForTime: yForMidiKey:

	initialization
	initialize on: updateLowestNote

	menu
	addCustomMenuItems:hand: addKeyboard addMovieClipPlayer beatLinesOnOff invokeScoreMenu: measureLinesOnOff

	scrolling
	autoScrollForX: goToTime: moveCursorToTime: notesInRect:

	stepping
	step stepTime

	submorphs-add/remove
	privateRemoveMorph:

	accessing	top
	beatsPerMeasure:
	movieClipPlayer
	movieClipPlayer:
	notePerBeat:
	score
	scorePlayer
	selection Returns an array of 3 elements: trackIndex indexInTrack of first note indexInTrack of last note
	selection:
	timeScale
	timeSignature:over:

	drawing	top
	addNotes Recompute the set of morphs that should be visible at the current scroll position.
	drawMeasureLinesOn:
	drawOn:
	drawStaffOn:
	isBlackKey: Answer true if the given MIDI key corresponds to a black key on the piano keyboard.
	rebuildFromScore Rebuild my submorphs from the score. This method should be invoked after changing the time scale, the color or visibility of a track, the extent of this morph, etc.

	dropping/grabbing	top
	acceptDroppingMorph:event: In addition to placing this morph in the pianoRoll, add a corresponding event to the score so that it will always appear when played, in addition to possibly triggering other actions

	editing	top
	appendEvent:fullDuration:
	copySelection
	cutSelection
	deleteSelection
	insertSelection
	insertTransposed

	event handling	top
	handlesMouseDown: Do I want to receive mouseDown events (mouseDown:, mouseMove:, mouseUp:)?
	mouseDown: Handle a mouse down event. The default response is to let my eventHandler, if any, handle it.
	mouseMove: Handle a mouse move event. The default response is to let my eventHandler, if any, handle it.
	mouseUp: Handle a mouse up event. The default response is to let my eventHandler, if any, handle it.

	geometry	top
	contractTime
	expandTime
	extent: Force rebuild when re-sized.
	fullBounds Overridden to clip submorph hit detection to my bounds.
	layoutChanged Override this to avoid propagating 'layoutChanged' when just adding/removing note objects.
	midiKeyForY:
	tickTimeAtCursor
	timeForX:
	xForTime:
	yForMidiKey:

	initialization	top
	initialize
	on:
	updateLowestNote find the actual lowest note in the score

	menu	top
	addCustomMenuItems:hand: Add morph-specific items to the given menu which was invoked by the given hand. This method provides is invoked both from the halo-menu and from the control-menu regimes.
	addKeyboard
	addMovieClipPlayer
	beatLinesOnOff
	invokeScoreMenu: Invoke the score's edit menu.
	measureLinesOnOff

	scrolling	top
	autoScrollForX: Scroll by the amount x lies outside of my innerBounds. Return true if this happens.
	goToTime:
	moveCursorToTime:
	notesInRect:

	stepping	top
	step Do some periodic activity. Use startStepping/stopStepping to start and stop getting sent this message. The time between steps is specified by this morph's answer to the stepTime message. The generic version dispatches control to the player, if any. The nasty circumlocation about owner's transformation is necessitated by the flexing problem that the player remains in the properties dictionary both of the flex and the real morph. In the current architecture, only the top renderer's pointer to the player should actually be honored for the purpose of firing.
	stepTime Answer the desired time between steps in milliseconds. This default implementation requests that the 'step' method be called once every second.

	submorphs-add/remove	top
	privateRemoveMorph: Private! Should only be used by methods that maintain the ower/submorph invariant.