LP#1102472: add about:about to staff client developer menu

The about:about page contains many XULRunner diagnostic tools.

Some useful ones include:

about:buildconfig - Get information about how the staff client is built.
about:config - Tweak XULRunner settings.
about:memory - Get detailed information about staff client memory usage.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>