Change documentsearch shortcut selector to searchable class

[aschlaep]

This addresses bullet 1 in #6217. When the active widget changes, the documentsearch extension checks if it is searchable and applies the jp-mod-searchable class if it doesn't have it already. Then the Accel F and other shortcuts are scoped to only this class, so that they'll pass through to the browser when the focus is outside a searchable widget.

@afshin @jasongrout

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[afshin]

Nice. Thank you!

👍

[afshin]

I thought about this PR some more. To make document search available to non-JupyterLab applications more easily, this ILabShell logic could be in its own JupyterFrontEndPlugin<void> so that the main search plugin that adds the commands installs without that hard dependency.

[jasongrout]

To elaborate for @aschlaep - this would just be splitting this one piece of logic involving the labshell into its own plugin. A test of how this makes things more flexible would be adding document search to the plain notebook example at https://github.com/jupyterlab/jupyterlab/tree/master/examples/notebook.

[aschlaep]

I moved the css class application to its own plugin and added documentsearch to the notebook example, with help from @jasongrout!

[blink1073]

Very nice, thanks!