Added a new subsistem for "complex" animations

Another tiny update for the game, there is no change in game mechanics but this time we added the first steps for a, hopefully, simple way to create complex animations. The new intro scene, in the menu section, is defined as follows

(def intro-screenplay
  [screenplay/action-after :time 1.5
   :then [screenplay/action-move
          :actor :ufo
          :from [0 0] :to [248 200] :time 5.0
          :then [screenplay/action-after
                 :time 3.0
                 :then [[screenplay/action-scale
                         :actor :cow
                         :from 0.1 :to 1.0 :time 4.0]
                        [screenplay/action-move
                         :actor :cow
                         :from [248 200] :to [248 400] :time 4.0]]]]])

With this new subsystem I hope to speed up the creation of cut scenes and other narrative game content; right now the only available actions are move, scale and after (which basically waits for some time); it is also quite generic and not too coupled with internal conventions but I am not planning to use other backend other than PIXIjs so I don't know for sure yet.