* app/core/core-enums.[ch]: add GIMP_ARRANGE_FOO values
	to GimpAlignmentType, and change CENTER to HCENTER,
	MIDDLE to VCENTER.

	* app/core/gimpimage-arrange.c: extensively rewritten
	to handle arrangement of objects, and to do the
	required sorting of lists by offset.

	* app/tools/gimpaligntool.[ch]: added ability to arrange
	groups of layers etc with constant spacing.  Also try
	to change things so that the tool aligns with the
	object that users expect intuitively.