Fix #1562, fix regressions in PerspectiveWidget

[sc1f]

This PR fixes some issues with PerspectiveWidget:

• Updates were broken in the RC release but are now working
• Client-server editing is now working again
• Use one shared_worker for all widget instances, instead of a new worker per-instance
• Update binder to 3.1.14, as Jupyterlab 3.0.x has a Webpack issue that causes a parse error when importing the Widget front-end.
• Remove the MutationObserver on widget attributes as the attributes are no longer fixed on the front-end

As written in #1560, the goal for the widget is for a complete rewrite that cleans up both the Python and the JS side, and removes any dependency on traitlets. However, this PR fixes the regressions between the 1.0.0 RC release and our last release.

[texodus]

Looks good! Thanks for the PR!