Add dropdownShown and dropdownHidden events.

They're a bit different with `opened` and `closed` events.
As I found, the `opened` and `closed` events are fired when the input gets the focus or not.
`dropdown...` events I implemented are fired when the dropdown element's `display` style is changed.

I used them to change the css of the input element, especially `border-radius` to flatten the border-bottom only when the dropdown is shown.
I think there may be others who want the feature, and I upload this pull request.

Thanks.