A selectionMorph supports the selection of multiple objects in a morphic world or pasteUp.

Structure:
selectedItems an OrderedCollection of Morphs
These are the morphs that have been selected
slippage a Point
Keeps track of actual movement between the
steps of gridded movement
dupLoc a Point
Notes the position when first duplicate request occurs from halo
dupDelta a Point
Holds the final delta of the first duplicate plus subsequent moves.

	drawing
	drawOn:

	geometry
	bounds: extent: privateFullMoveBy:

	halo commands
	addCustomMenuItems:hand: addHandlesTo:box: addOptionalHandlesTo:box: addOrRemoveItems: balloonHelpTextForHandle: borderColor: borderWidth: dismissViaHalo doDup:fromHalo:handle: fillStyle: maybeAddCollapseItemTo:

	initialization
	extendByHand: extendByHandPlus: hasHalo: initialize

	private
	doneExtending selectSubmorphsOf: selectedItems setOtherSelection: setSelectedItems:

	undo
	borderColorForItems: borderWidthForItems: fillStyleForItems: refineUndoTarget:selector:arguments:in: undoMove:redo:owner:bounds:predecessor:

	drag and drop	top
	aboutToBeGrabbedBy: The receiver is being grabbed by a hand. Perform necessary adjustments (if any) and return the actual morph that should be added to the hand.
	justDroppedInto:event: This message is sent to a dropped morph after it has been dropped on -- and been accepted by -- a drop-sensitive morph

	drawing	top
	drawOn:

	geometry	top
	bounds: Make sure position: gets called before extent:; Andreas' optimization for growing/shrinking in ChangeSet 3119 screwed up selection of morphs from underlying pasteup.
	extent:
	privateFullMoveBy: Private! Relocate me and all of my subMorphs by recursion. Subclasses that implement different coordinate systems may override this method.

	halo commands	top
	addCustomMenuItems:hand: Add custom menu items to the menu
	addHandlesTo:box: Add halo handles to the halo. Apply the halo filter if appropriate
	addOptionalHandlesTo:box:
	addOrRemoveItems: Make a new selection extending the current one.
	balloonHelpTextForHandle: Answer a string providing balloon help for the given halo handle
	borderColor:
	borderWidth:
	dismissViaHalo The user has clicked in the delete halo-handle. This provides a hook in case some concomitant action should be taken, or if the particular morph is not one which should be put in the trash can, for example.
	doDup:fromHalo:handle:
	fillStyle: Set the current fillStyle of the receiver.
	maybeAddCollapseItemTo: ... don't

	initialization	top
	extendByHand: Assumes selection has just been created and added to some pasteUp or world
	extendByHandPlus: Assumes selection has just been created and added to some pasteUp or world
	hasHalo:
	initialize

	private	top
	doneExtending
	selectSubmorphsOf:
	selectedItems
	setOtherSelection:
	setSelectedItems:

	undo	top
	borderColorForItems:
	borderWidthForItems:
	fillStyleForItems:
	refineUndoTarget:selector:arguments:in: Any morph can override this method to refine its undo specification
	undoMove:redo:owner:bounds:predecessor: Handle undo and redo of move commands in morphic