Implementing datasets panel skeleton

[martinRenou]

Fix #17

Populate the left panel with current datasets + add menu and commands logic for creating new viewers

[github-actions[bot]]

:point_left: Launch a Binder on branch _martinRenou/glue-lab/datasetspanel

[martinRenou]

Turning this into draft again. I want to add a bit more to this.

[martinRenou]

Thanks both! I took your comments into account

[martinRenou]

I also changed the implementation to use a sub-menu:

[trungleduc]

Thanks @martinRenou