custom widgets support for notebook outputs

[jonah-iden]

What it does

this adds support for other ipywidget types like ipydatagrid and ipyleaflets.
Also fixes focusing of notebook editor widget after its shown and updating associated notebooks for extensions after the workspace was granted trust.

To explain overwriting the append child of head in output-webview.internal.ts a bit more:
Vscode has a single endpoint all webviews can downlaod script files from, in theia though every webview can only download files from its own host address as the endpoint.
There is a asWebviewURL function in the extension API which we don't know for which webview the webviewUri is for. In VScode thsts no problem because the endpoint is always the same but in theia its dependent on the webview.
So what im doing is adding a placeholder to the url which is replaced when a script is added to the head to generate the correct URL and make downloading the script work.

How to test

pip install ipydatagrid in your used python environment
Create or open a notebook and add the following cell.

import numpy as np
import pandas as pd
from ipydatagrid import DataGrid

df = pd.DataFrame(
    data={
        "Col1HasAVeryLongName": [1, 2, 4],
        "Col2MediumName": [4, 5, 6],
        "Col3": [7, 8, 10],
    }
)
grid = DataGrid(df, index_name="index_column", layout={"height": "90px"})
grid

Follow-ups

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[jbicker]

Tested in browser version. Works!
LGTM.