"Editable" in metadata should not allow cell transformation

[katsar0v]

Describe the bug
There is the not documented feature that puting {'editable':false} in the metadata prevents the cell from being edited. Unfortunately this works only for cell content, still the cell can be transformed, e.g. from code to markdown or raw to code (via dropdown or shortcode).

Expected behavior
I guess putting this value in the metadata should stop the cell from being modified in any form.

Desktop (please complete the following information):

• OS: Ubuntu 18.04
• Browser - Chrome 73.0
• JupyterLab - 0.35.4