* 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.
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