Revert "Manipulator: remove unused event hack"

This reverts commit 134e927965.

Writing into const event is very bad,
but this change broke compositor manipulators.

Will look into better solution eventually.

source/blender/windowmanager/manipulators/intern/wm_manipulator_group.c

@@ -362,6 +362,9 @@ static int manipulator_tweak_modal(bContext *C, wmOperator *op, const wmEvent *e
 		return OPERATOR_FINISHED;
 	}
 
+	/* Ugly hack to send manipulator events */
+	((wmEvent *)event)->type = EVT_MANIPULATOR_UPDATE;
+
 	/* always return PASS_THROUGH so modal handlers
 	 * with manipulators attached can update */
 	return OPERATOR_PASS_THROUGH;

source/blender/windowmanager/wm_event_types.h

@@ -338,6 +338,9 @@ enum {
 	EVT_MODAL_MAP         = 0x5022,
 	EVT_DROP              = 0x5023,
 	EVT_BUT_CANCEL        = 0x5024,
+
+	/* could become manipulator callback */
+	EVT_MANIPULATOR_UPDATE     = 0x5025,
 	/* ********** End of Blender internal events. ********** */
 };