feat(collapse): add eventemitter

[otelnov]

• collapsed() - An optional expression called after the element finished collapsing.
• collapsing() - An optional expression called before the element begins collapsing.
  If the expression returns a promise, animation won't start until the promise resolves.
  If the returned promise is rejected, collapsing will be cancelled.
• expanded() - An optional expression called after the element finished expanding.
• expanding() - An optional expression called before the element begins expanding.
  If the expression returns a promise, animation won't start until the promise resolves.
  If the returned promise is rejected, expanding will be cancelled.

[Dinistro]

I would work with Observables and not with Promises.