Clean up how widgets added during SDI mode are rehydrated after toggling to MDI.
Clean up how widgets added during SDI mode are rehydrated after toggling to MDI.

eskirk/jupyterlab,charnpreetsingh185/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab,charnpreetsingh185/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,eskirk/jupyterlab,jupyter/jupyterlab,eskirk/jupyterlab,jupyter/jupyterlab,eskirk/jupyterlab