Allow focus changes from Update

[xarvic]

This PR adds the methods needed to change focus to UpdateCtx.

[jneem]

What's the status of this? I don't see anything wrong with it, but was it discussed somewhere?

[cmyr]

I have reservations about this. It's possible that it's okay, but when events are triggered during update the interactions are complex.

I think for the time being I would prefer to handle this by dispatching a command; it isn't elegant but it is easier to understand the control flow.

[xStrom]

was it discussed somewhere?

I think the genesis is in this zulip thread.

[xStrom]

What's the status of this PR? @xarvic do you think we should still move forward with this?

If we should move forward, then please rebase on master and I will try to figure out if this change makes sense.
If you no longer think this is relevant then we can just close this PR.