Notify about connection lost and reconnection events

[mattpap]

I improved ConnectionLost event to contain all the necessary information and added reconnect() method, which can be used to manually reconnect broken connection. I intend to add a very basic notification scheme in this PR to allow UI feedback for ConenctionLost and ClientReconnected events.

fixes #14564
addresses #14252

[mattpap]

I added very rudimentary support for connection notifications, e.g.:

I still need to do some tweaking, but I think this should tentatively ready for review.

[mattpap]

@philippjfr, if you want to use panel's notifications instead of this built-in approach, then I will need to add somewhere an option to allow disable notifications.

[philippjfr]

Just to record what we just discussed. It would be good to have a mechanism to disable this. A BokehJS setting is fine for now but would be nice to expose it from Python eventually.

[mattpap]

@philippjfr, you can now use:

import {settings} from "core/settings"
settings.notifications = false

[mattpap]

A BokehJS setting is fine for now but would be nice to expose it from Python eventually.

As mentioned in the meeting, I intentionally didn't expose any public APIs this late in 3.8. I will do that for 3.9.

[philippjfr]

I'm happy with this now. I'll have to spend some time integrating this in Panel, and if I uncover issues there I'll open an issue. Let's get this merged and then tag a dev release.

[mattpap]

@philippjfr, I started 3.8.0.dev4 build, it should be ready in an hour.

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