added return values to addClassName and removeClassName to let the caller know if the style list was changed or not.
extract common indexOfClassName method for finding a style if it exists.

Review at http://gwt-code-reviews.appspot.com/1873803


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11406 8db76d5a-ed1c-0410-87a9-c151d255dfc7