The editModes are set externally by the parent dialog box.
The same goes for which bitmaps are visible.

tile mode:
	Clicking replaces the tile in that location, 
	with the tile currently selected in the "tilePalette" pane.
	It also replaces the terrain value in that location with the
	default terrain value of the selected tile.

terrain mode:
	Clicking replaces the terrain value in that location, 
	with the terrain value selected in the "Terrain Type" DropList.

event mode:
	Clicking selects the location, placing a square cursor there.
	Pressing SPACE Edits / Creates a new event in that location,
	and opens the event editor.
	
	SPACE:	Edit / Create event
	DELETE:	Delete selected event
	ARROWS:	Shifts the "eventCursor" in that direction & selects that tile.
			The cursor cannot leave the bounds of the map.
	MouseDrag:	Tenatively relocates selected event.
			The "dragIcon" appears with the mouse cursor.
			and the "eventIcons" bitmap has the selected event temporarily erased.
			Disables keyboard commands.
	MouseDrop:	Re-locates the dragged event to the current tile. (if possible)
			The "dragIcon" vanishes.
			Re-enables keyboard commands.
			If there's already an event on this tile, the procedure is cancelled.
			Success:
				The event is assigned to the new location.
				The event icon is drawn on the "eventIcons" bitmap at the NEW tile.
			Fail:
				The event is un-changed.
				The event icon is re-drawn on the "eventIcons" bitmap 
				at its original tile.

selection mode:
	Clicking selects the location.