Changed EditorNode3DGizmoPlugin::current_gizmos from List to HashSet, to avoid having to iterate through all gizmos when ~EditorNode3DGizmo unregisters itself.
Random-access access to `List` when iterating is `O(n^2)` (`O(n)` when
accessing a single element)
* Removed subscript operator, in favor of a more explicit `get`
* Added conversion from `Iterator` to `ConstIterator`
* Remade existing operations into other solutions when applicable
