app: use the new modifier API instead of constants in all tools

There are still many uses of literal SHIFT and MOD1 left, but all uses
of CONTROL are gone. Should work exactly as before on Win/X11, and
still has some glitches on OSX.

app/tools/gimprectangletool.c

@@ -40,6 +40,8 @@
 #include "core/gimppickable.h"
 #include "core/gimptoolinfo.h"
 
+#include "widgets/gimpwidgets-utils.h"
+
 #include "display/gimpcanvasgroup.h"
 #include "display/gimpdisplay.h"
 #include "display/gimpdisplayshell.h"
@@ -1278,7 +1280,7 @@ gimp_rectangle_tool_active_modifier_key (GimpTool        *tool,
         }
     }
 
-  if (key == GDK_CONTROL_MASK)
+  if (key == gimp_get_toggle_behavior_mask ())
     {
       g_object_set (options,
                     "fixed-center", ! options_private->fixed_center,