Add the variable inspector to the right area

[jtpio]

Fixes #137

As suggested in #137, the right sidebar sounds like a nice place for the variable inspector widget:

right-sidebar.mp4

It's also possible to switch sidebar side if needed:

switch-sidebar.mp4

Changes

• Add the variable inspector to the right area by default
• Remove the command to open a new inspector in the main area

[jtpio]

As a nice side-effect, this extension will probably be compatible with the next major version of the Jupyter Notebook 7 as well:

Probably a couple of things to address so the context is properly detected, but could be fixed separately.

[lckr]

I think this look very nice but breaks with the matrix viewer that displays numpy arrays or similar.
I'm currently trying to get around this issue and also use a proper SidePanel to host the variable inspector in.

[jtpio]

cc @ericsnekbytes since you said you were looking into this.

Maybe this PR might not be needed with the layout customization

• Customize the shell layout with the settings jupyter/notebook#6921
• https://jupyterlab.readthedocs.io/en/latest/user/interface_customization.html#layout