* app/core/core-enums.[ch]: add GimpAlignReferenceType enum

	* app/core/gimpimage-arrange.c: support using a channel as
	reference for alignment.

	* app/tools/gimpaligntool.[ch]: support aligning with a list item,
	image, selection, active layer, active channel, or active path.
	Now similar in form and function to Inkscape Align/Distribute
	dialog.