POC: sidebar widget plugins

[kecnry]

This is a proof-of-concept for a sidebar widget plugin infrastructure in which the plugins:

• have separate API/state and UI, with the API accessible from the notebook
• can be context-aware and determine whether they should be included in the UI or not

This also enables accessing the glue-state from Jupyter with:

from glue_solara.app import AppAPI
app = AppAPI()
app.glue_state
app.plugins
app.show()