Trivial stub installer cleanup - resized the dialog, pointed at final URL for...

Trivial stub installer cleanup - resized the dialog, pointed at final URL for slideshow, added final art, fixed UserAgent.  Added 1-line comment to llstartup.cpp about updater URL.  Going into trunk because it's not tied to a viewer release and the functional code has been through QA elsewhere.  OK by Steve/Q.  Reviewed with Kelly.

svn merge -r113629:113831 svn+ssh://svn.lindenlab.com/svn/linden/branches/windows-setup/windows-setup-4