* app/display/gimpdisplayshell-callbacks.c: allow modifier key events
	for Shift and Control to be propagated even if button1 is down.

	* app/tools/gimpclonetool.c
	* app/tools/gimpcroptool.c
	* app/tools/gimpforegroundselecttool.c
	* app/tools/gimpmovetool.c
	* app/tools/gimppainttool.c
	* app/tools/gimpvectortool.c: change  modifier_key method
	to prevent any bad consequences.

	* app/tools/gimprectangletool.[ch]:  add modifier_key handler, and use it
	to toggle "make-square" if Shift is pressed while button1 is down, and
	"fixed-center" if Control is pressed while button1 is down.

	* app/tools/gimprectangleselecttool.c (gimp_rectangle_tool_modifier_key):
	call rectangle tool modifer_key method after chaining up.