Adding a couple of useful methods to AbstractCellTable.  getColumnWidth() can be used to access the width of a column set using setColumnWidth().  flush() will immediately flush pending changes into the view.

Review at http://gwt-code-reviews.appspot.com/1453801

Review by: pengzhuang@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10259 8db76d5a-ed1c-0410-87a9-c151d255dfc7