Visualizer mappings are lost when grouping nodes

[PathogenDavid]

This is something that came up when Gonçalo and I were discussing #1769

Starting with the following workflow: LostVisualizerMapping.zip

Right-click the Format node and select Group > GroupWorkflow. The node will be grouped, but the visualizer mapping association will be lost:

Ideally one of two things should happen:

1. Bonsai should refuse to group the node because there's a visualizer mapping
2. The visualizer mapping should be associated with the GroupWorkflow after the grouping is complete

Option 2 makes the most sense in this specific scenario, but consider a situation where the mapping is not to the final node in the selected subgraph. Where would the visualizer mapping go to in this scenario? Mapping it to the resulting group would have different semantics.

We could do option 2 when possible and fall back to a warning when it's not, although it might be better to have the user do it manually so they don't end up in a situation where they don't realize they're changing semantics of the outer visualizer mapping when they change the nodes at the output of the subworkflow. (This of course is an problem that must be considered even when the user does this manually, that may be a moot point.)