desktop_browser: fix wait_still_screen, drop wait_idle
    
    wait_idle was deprecated and effectively it just sleeps, so
    replace these wait_idles with explicit sleeps - I can't think
    of a better choice here as we just need to wait for Firefox to
    calm down after various operations.
    
    The wait_still_screens were always hitting the 30 second time
    out because of the flashing cursor; setting the similarity level
    required to lower than the default 47 should fix this.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>