Make sure Fill uses undo (but also intelligently)

"Fill", as a new main-toolbar tool, was not recording snapshots
of the image for the "Undo" tool.  Mended.

However, also updated the tool so that it doesn't _bother_ recording
into the undo buffer if the fill action is a no-op (e.g., clicking
the same spot a second time, or otherwise attempting to fill an
area with a color that's identical to what's already on the canvas).

docs/CHANGES.txt

@@ -13,7 +13,10 @@ $Id$
     ---------
     * Fill
       Not actually a new tool, but promoted from a "Magic" tool
-      to a full-fledge tool in the "Tools" bar.
+      to a full-fledge tool in the "Tools" bar.  However, also
+      avoids filling "Undo" buffer with snapshots of the drawing,
+      if the fill does nothing (e.g., if you click the same spot
+      a second time).
 
   * Ports & Building
     ----------------