Dynamic overlay

[philippjfr]

This PR makes it possible to use return Overlays with changing keys in a DynamicMap callback. It's a sensitive change, which will require some more testing.

Note that while it now displays everything certain things do not yet work:

• Legends do not yet change to reflect the new NdOverlay key or Overlay label
• In matplotlib styles will not consistently update.

And certain things cannot work:

• Cyclic styles cannot work with a DynamicMap as the total ordering is required during instantiation. Could consider raising a warning when an Element returned from a DynamicMap is given a cyclic style.
• You cannot return more Elements or different types of Elements than are supplied during instantiation.

[philippjfr]

We've opened some issues to improve the validation for DynamicMap return types and warning the user. For now though this PR is ready for merge.

[github-actions]

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