notebook: Remove celltoolbar if not notebook
notebook: Remove celltoolbar if not notebook

If the current Panel is not a NotebookPanel, remove the cell toolbar tab
from the left menu. Show it only when the Panel is a NotebookPanel.

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