transformtool: Add temporary new handles

put perspective handles on the outside instead (if you invert the frame,
they'll switch place with the scale handles, oh well)

app/display/display-enums.h

@@ -58,6 +58,8 @@ typedef enum
   GIMP_HANDLE_FILLED_SQUARE,
   GIMP_HANDLE_CIRCLE,
   GIMP_HANDLE_FILLED_CIRCLE,
+  GIMP_HANDLE_DIAMOND,
+  GIMP_HANDLE_FILLED_DIAMOND,
   GIMP_HANDLE_CROSS
 } GimpHandleType;