Add mnemonics for DevMode swing UI.

Original review at http://gwt-code-reviews.appspot.com/708801/show

Review at http://gwt-code-reviews.appspot.com/1328801

Issue: 5150
Patch by: alexander.orlov, jat
Review by: jat, conroy


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9648 8db76d5a-ed1c-0410-87a9-c151d255dfc7