Adds support for background color and background image to Scamper. This is
an ALPHA version, it works, but I've tested it only for 15 minutes.

Possible to-do: download the background image through DownloadingImageMorph.

Links:
http://fce.cc.gatech.edu/~bolot/swiki.php3?folder=squeak/scamper
http://fce.cc.gatech.edu/~bolot/squeak/scamper/Scamper-Background.3.cs