Add update page title command

[richard-to]

This change adds a command that can update the page title.

A few notes:

• Turns out we set the page title after every state change, so a web component wouldn't have worked here
• One issue I ran into design-wise is that we need to keep the state of the updated page title. Ideally, it seems like it sent with the UI state every time. In the end, I chose not to do that. Instead just stored this client-side state as a variable on Channel class for now, which I'm not sure is ideal. But I think can be refactored later if we get more uses cases where we have state that is separate from the main mesop state object. I guess we do have query params and it's possible I could have used a similar but simplified pattern for storing the page title.

Closes #331

[wwwillchen]

LGTM - a comment around the naming, otherwise looks good - thanks!

[wwwillchen]

(no action needed) food for thought: it's a little weird to me the interaction between channel and shell (which has been a while for a long time) - they both want to call each other (which would be a cyclic dependency) so we use this indirection.

I think this PR doesn't add that much incremental complexity, but perhaps in the future we should refactor shell and channel. It seems like channel is doing too much (it's both the transport layer + a stateful rendering service) and could be split up into 2+ classes (although the exact split is kind of unclear to me now).

[richard-to]

Yeah, I agree about this. I'll keep this in mind for the future.