Support for existing Jupyter lab widgets

[drafter250]

Amazing work on the 0.5.0 release! I know flexx can be used in notebooks and maybe jupyter lab( or at least soon), is there a canonical method for using jupyter lab widgets in flexx? If you look at what some people have built https://jupyter.org/widgets especially the 3d stuff, it would be great to be able to just use those in a full blown flexx app!

[almarklein]

There has been some work on integration between Jupyterlab and Flexx, but a few things have changed since then:

• Jupyterlab does no longer allow insertion of <script>, thereby breaking Flexx' way to inject itself in the notebook. It means that Flexx now needs an extension to work in the Jupyterlab notebook.
• Flexx no longer depends on PhosphorJS. Actually I'm not sure whether Jupyter widgets do, since they also work in the normal notebook? Anyway, I am not sure how hard it would be. Or how valuable; Jupyter widgets seems to have similar goals to Flexx, but tight to Jupter.

Things like making Flexx work in Jupyterlab is something that would be a nice project for a contributor; I don't have an intensive to work on this.