Fixed Panels to update height when contents change.

This addresses #207 - CollapsableMixin now sets the dimension style attribute to 'auto' when the element state has finished changing from collapsed to expanded.
The use of that function is odd in itself, because it looks like state in this mixin is never updated to reflect the expanded state.

Also notice that in the _removeEndTransitionListener func it is actually adding the event listener again, which I don't entirely understand. It works, but why it is written that way is unclear from the code.