gui doesn't have join feature

the objmc cli has the `--join` argument which allows user to join together multiple different objmc models that may have different settings.

this allows combined models such as:
- a humanoid with body set to autorotate yaw and head set to autorotate both yaw and pitch, with separate animation controls
- models with different visibility settings for gui/hand/world so that a different model is visible depending on location
- models with different rotation colorbehavior bits so that they rotate separate ways when changing color
- etc...

this feature is currently not accessible in the gui

- how can i properly communicate the function of the feature properly with the interface?
- how can i make it easy to use while letting user create/modify multiple models with different settings?