Add position to pulic API

[patric-eberle]

I think it would be helpful to have the current position of the dropdown in relation to the trigger as a computed property on the public API.

In my case I would like to style the trigger based on the position of the dropdown, but this seems currently not possible without using open/close or calculate the position myself.

e.g. this.get('dropDownInstance.position') could return a string left:top or an object { horizontal: 'left', vertical: 'top' }