Replaced uses of AlphaImageLoader in GWT style themes with references to single-bit transparent image files.  IE7 now uses the multi-bit (anti-aliased) image files that it supports.  IE6 uses single-bit (hard edged) files, although the difference is barely noticeably.

Patch by: jlabanca
Review by: ecc (TBR)
Issue: 3278

git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.6@4487 8db76d5a-ed1c-0410-87a9-c151d255dfc7