Add destructor to <messagecatalog>

Add destructor to <messagecatalog>

And move <help> binding to separate file and just have menu_frame.xul load it
for now.  This really reduces the initial memory footprint of the staff client,
but it doesn't addresses all leaks--after having watched the process list after
many interations since I don't quite grok all the debugging tools.  I wanted to
try replacing messagecatalog with the native stringbundle, which Galen confirmed
works with the oils:// protocol, but I ran into some trouble.  That might be
another experiment worth pursuing.

Signed-off-by: Jason Etheridge <jason@esilibrary.com>