Add onClear event to dropdown

[ajsaraujo]

I'm submitting a ...

• bug report
• feature request
• support request

The dropdown component has a showClear input - when set to true, it shows a little ❌ sign that you can click to reset the dropdown. It is very handy. You know what would be even nicer? An onClear event emitter. With it, we would be able to easily do something every time this button is clicked.

My use case: I have two dropdowns, and when one is cleared, the other should be as well. Of course, I could listen to onChange and check if the new value is falsy, but a onClear event would be more semantic.