PhosphorJS status tracking

[robobenklein]

Tracking eclipse-theia/theia#6501

Doesn't look like https://github.com/jupyterlab/lumino includes any of the CR-required additions like save/restore layouts for non-Dock types of panels, think they'd be interested in those features getting upstreamed? Also still need to investigate if it's any better at providing drag/drop/hover/data events than the current phosphor.

IBL/IBP and similar might even be classified as upstreamable, not sure who else would use them yet though.

[robobenklein]

>= 1.60 is now open to migrate to lumino, notably changes to the application shell, dragging, and need to check on derives for the CR Box/Strip/Patch subclasses

https://github.com/eclipse-theia/theia/pull/14320/files#diff-cb4b87cc3dc274963d24f6fd5682ad9c89cd51d94726100eb2e3c92421e5c171