TrackballControls: Refactor API.

[Mugen87]

This PR makes the API more similar to OrbitControls.

[WestLangley]

The only significant difference between OrbitControls and TrackballControls should be the update() method.

Rather than changing the API at this point, I suggest freezing that file, and extending OrbitControls to accommodate TrackballControls -- just as we did with MapControls. That way they will all have the same API.

Edit: It works.

[Mugen87]

The API of TrackballControls will change whatever we are going to do. Merging this PR or merge TrackballControls with OrbitControls.

Hence, I see no problem to merge this now since there will be a migration effort (examples, docs etc.) in any case.