WIP: Shape controls -- center vs corner

Adding control buttons to the Shapes tool, allowing for shapes
to be drawn from the center (as before) or from a corner
(more like other paint packages).  The controls do nothing
at this time, but are visible and can be clicked.  This is a
work in progress.

The controls may be removed using a configuration option.

src/parse.h

@@ -15,6 +15,7 @@ struct cfginfo
   const char *datadir;
   const char *disable_label;
   const char *disable_magic_controls;
+  const char *disable_shape_controls;
   const char *disable_print;
   const char *disable_quit;
   const char *disable_save;