Bug 1037316 part 1 - Return the same object when updating animations; r=dbaron

Previously when updating animations we'd generate a new list of animation
objects then try to match up animations from the existing list and copy across
state such as start times and notification flags. However, this means that from
the API we end up returning different objects.

This patch makes us maintain the same object identity when updating an existing
animation. It does this by looking for matching animations in both lists. If it
finds a match it copies the necessary information from the *new* animation to
the *existing* animation (but preserving the start time, last notification
etc.). Then, finally, it puts the *existing* animation in the list of *new*
animations and removes the corresponding *new* animation. The existing
animation is also removed from the list of existing animations so that it only
matches once.

The method used for matching is probably not intuitive but this is addressed in
a subsequent patch in this series.
Bug 1037316 part 2 - When updating animations, match existing animations one-by-one starting from the beginning of each list; r=dbaron

This patch changes the order in which we look for matches when updating existing
animations. Previously we would iterate through new animations in a forwards
direction but match old animations by going through the list of animations
backwards.

This patch makes us iterate through both lists in a backwards direction. That
means that if we have:

  animation: anim 100s

and later we make it

  animation: anim 100s, anim 100s

Then the new animation will be added to the *start* of the list, i.e. prepended,
and the resulting animation will not restart.