Deprecate the frame definition workflow UI

[adamkewley]

This is a stub issue for deprecating the frame definition UI. This is where users will be directed if they want it to be un-deprecated.

The currently-proposed timeline for deprecation is:

1. Late 2024: announce it's deprecated and add an appropriate label into the UI. The label should mention this issue, so that users know where to go if they want it to be kept alive.
2. Allow 5 or 6 releases (5-6 months) of OSC to circulate with the label and collect feedback from users.
3. Remove the link to the frame definition UI from the splash screen. Otherwise, keep it in-codebase. Make it only accessible via the "Experimental Tabs" section of the UI, and add a warning/popup when it's opened that directs users to this ticket.
4. Wait for 5 or 6 more releases of OSC to circulate with that label etc.
5. If noone seems to care, drop it from the codebase late-2025

The reason it's deprecated is because it turned out that it was easier to just merge StationDefinedFrame into opensim-core itself, which means that the model editor UI now technically supports creating frames that way.

In general, I want OSC to gradually move towards having one workflow that caters to 95 % of requirements by (e.g.) merging the editor, preview experimental data, and simulator tabs into one workflow, rather than the current situation of there being 6 or so workflows for each sub-task of model building. The editor should also provide much more flexibility for moving things around so that we can also eventually deprecate the mesh importer.

The frame definition workflow is first on the hitlist because it's undocumented and I can't imagine users use it all that much. If users think otherwise, then this issue is the place to discuss what's possible (not deprecating it, making the model editor support useful features from it, etc.).