Try to make desktop_browser more robust
    
    We've been getting failures lately on the first page load, I
    think because Firefox is getting even more grindy on startup. So
    turn the 'sleep' into a 'wait_still_screen', extend another wait,
    and tweak the 'browser' needle so it only matches after the
    bookmark bar has loaded rather than as soon as half the chrome
    appears. Also make all the wait_still_screens use similarity 45
    for consistency (flashing cursor could be there on any of them).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>