Add iron-collapse-resize event on transitionEnd

[cedric-marcone]

In 0.5 a core-resize event was triggered once the transition ended.
It was super useful if you wanted to chain something once the animation was finished.

This PR restores the event but I renamed it iron-collapse-resize instead of iron-resize to prevent collisions using the element name as a namespace.
I don't know if it's the canonical way though :)

[cedric-marcone]

Any chance of discussing this PR ? I'm currently stuck with my own version of the element. My only alternative, as of today, if I want to use the original element, is to resolve a promise in an async call with a fixed delay. It may be you recommendation though.

[notwaldorf]

Sorry about this! I completely missed this PR, so nobody was actually looking at it.

I talked to @cdata and we think a better idea might be having iron-collapse use the IronResizableBehavior. That was has a notifyResize method which fires an iron-resize event for all the interested parties. What do you think?

[cedric-marcone]

Thanks, it seems to be a much saner approach and it was battle tested in core-collapse previously ;)