Adding MultiSelectionModel#clear() to clear all selected values. This is slightly more complicated because we resolveChanges() in a finally command, but the JavaDoc explains how we handle this.

Review at http://gwt-code-reviews.appspot.com/1089801

Review by: rchandia@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9201 8db76d5a-ed1c-0410-87a9-c151d255dfc7