[vue] emit opened/closed events in popover

[jesusantguerrero]

Hello, thanks for the work in this amazing library. I have a suggestion:

A common pattern in Vue app development is to relay in props down/events up to communicate between children and parent elements.

With that said it would be a nice addition to the package to throw these events (@opened/closed) when we just need to know when they happen and not to call them imperatively.

I would be happy to help with that.