DragControls: Refactor API.

[Mugen87]

Related issue: #29072, #20575

Description

This PR refactors the API of DragControls.

• THREE.Controls is introduced which defines a basic control interface. DragControls is now derived from this class.
• Side effects in the ctor are moved into a connect() method.
• domElement is now optional. If required, an instance of DragControls can now be created without a DOM element. When the element is set at a later point, a manual call of connect() configures the event listeners.

[puxiao]

Do you think it is appropriate to add saveState():xxx, setState(xxx), and reset() to Controls?

[Mugen87]

Not all controls support these methods so I would revisit this after all classes has been updated.

If we add these methods to the Controls interface, we should make sure all derived classes implement them. They are useful in any event.

[mrdoob]

I think we should move Controls.js to src. Maybe src/extras?

[Mugen87]

Sounds good!

[mrdoob]

@Mugen87 What do you think about being able to pass the dom element here?

That way the code could look like this:

const controls = new OrbitControls( camera );
controls.connect( renderer.domElement );

I guess we'll need to do a bit of management though:

connect( element ) {

    if ( this.domElement !== element ) {

        this.disconnect();

    }

    this.domElement = element;

}

[Mugen87]

As a convenient feature, I guess it's fine.

That said, domElement is public so devs can basically achieve the same without further changes to the control classes. However, the additional management code in connect() could prevent mistakes when devs want to exchange the DOM element (not sure how common this use case is though).

[mrdoob]

@Mugen87

constructor( object, domElement = null ) maybe?

Right now it's undefined by default but subclasses set it to null...