Use implicit animations instead of Tweener
    
    gnome-shell replaced Tweener with implicit animations, which affects
    us as we are re-using the Background animation time: All animations
    are now defined in milliseconds instead of seconds, so we are now
    off by a factor of 1000.
    
    But instead of adjusting the time, switch to implicit animations
    like gnome-shell.
    
    https://pagure.io/background-logo-extension/pull-request/16