Support for "box fill".

File format extension:

    <boxfill color left top right bottom></boxfill>

This is just an interim format until true polyfills can be implemented;
it should be easily to convert boxes into polygons. But boxes are
very easy to draw!

Infelicities:

    - The Glade file was not updated with the interface
      changes; I manually inserted the changes myself.

    - The icon is kind of wrong; it's the media stop icon
      and we ought to make a proper icon.

    - Presently select and toggle color does not change color;
      I'm amenable to changing this behavior.

    - No UI support for transparencies.

    - No "Default Box Fill" menu option; revert to "Default" does
      not work.

PDF and Cairo generation has been tested.  Button 2/Button 3 support not
tested.  Some minor refactorings included.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>