app: fix modifying the selection while GimpRectangleSelectTool is active

if did revert to the previous selection and thus break stuff like
enbaling quick mask or inverting the selection, because I merged the
undo magic it does into gimp_rectangle_select_tool_halt(), whereas
before it was done by the former gimp_rectangle_tool_cancel(), so only
on explicit cancel not HALT from whatever source.

Do the same in the new code and move the undo magic from halt() to
rectangle_response(CANCEL), which is exactly the same distinction as
with the old GimpRectangleTool code.