Allow draggable groups

[voigta]

This PR allows groups to be draggable. Currently there is no effect when we call group.draggable().

As noted in the documentation of DragControls, this only works if the scene contains a single draggable group. This might lead to some confusion but ignoring the .draggable() call for groups is also confusing...😅 At least setting the transformGroup property to true only affects the behavior for groups and not for other objects.

[falkoschindler]

Thanks for the pull request, @voigta!

The method .draggable() doing nothing on groups is indeed unexpected. So I tested your fix with the following example:

with ui.scene() as scene:
    sphere = scene.sphere().draggable()
    with scene.group().draggable():
        scene.box().move(x=1)
        scene.box().move(x=-1)

But just as mentioned in the documentation, the boxes are only draggable if the sphere isn't draggable. This isn't nice either.

I'll have to think about it. Maybe there's another way to get a more intuitive behavior. The drag handler in JavaScript could try to propagate the movement to the next group in its hierarchy. Otherwise raising a NotImplemented exception in group.draggable might be the better choice.

[falkoschindler]

We see the following options:

1. Raise a NotImplement exception when calling draggable() on a group as a temporary workaround to avoid confusion.
2. Raising a RuntimeError when calling draggable() if there are other draggables defined causing a conflict.
3. Somehow solving the problem, e.g. by dynamically updating the transformGroup on mouse down.

What do you think, @voigta?

[voigta]

Thanks for the feedback @falkoschindler.

Solving the problem (i.e. option 3) would definitely be the cleanest solution.

There have been updates of DragControls.js in three.js r162 that allow multiple draggable groups. However, the situation of a draggable group and a draggable object is still not nicely handled in the official repo. Therefore, I've added a small modification that should fix this problem and your example is now working as expected.

[falkoschindler]

Thanks for the update, @voigta!

The new solution looks really neat and it behaves just as expected! What a nice incidence that we just upgraded three.js.
I just moved the change into our npm.py script, so that it is automatically applied when updating third-party libraries.

Let's merge! 🚀