Move GimpRectangleSelectTool's image undo/redo logic to
GimpSelectionTool, by adding a pair of
gimp_selection_tool_{start,end}_change() functions.  These
functions should be called by subclasses before/after changing the
selection, having the functions take care of undoing/redoing the
existing selection as necessary.  This allows us to reuse this
logic in other selection tools, specifically, the free-select tool.