[orx-gui] Implement drop down menu for mutableList

I want to be able to display a mutable list in a dropDown. It should be possible to add and remove entries (then somehow refresh the dropdown).

This would allow to create a gui that allows creating and deleting instances of objects.

The current drop-down gui component only displays enum entries, which can't be modified during run-time.

I am willing to submit a PR to implement this change, but would need some guidance.

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