dropdown: open/close events should only be fired if necessary

[jnizet]

Currently, close events are emitted everytime the dropdown is closed, even if it's in fact already closed. In particular, since the dropdown adds a click listener to the document to close itself when a click happens outside the dropdown, a close event is emitted by the dropdown every time we click anywhere on the document.

The same goes for open events: an event is emitted when the dropdown is opened, even if it's already open.

This doesn't look right to me.

[pkozlowski-opensource]

@jnizet good catch, this is definitively a bug. Opened the #695 for this.