Remember focus in IDE panes

[Spacerat]

With this PR, the sidebar manages which elements are automatically focussed when switching to a new pane inside the IDE. Specifically:

1. When an element in a pane is focussed on, it is recorded as the pane's last focussed element.
2. When switching back to the pane: if there was a saved element, it is refocussed. (second PR commit)
3. If there is not, the first element with an 'autofocus' attribute is focussed. (first PR commit)
4. If there is no autofocus element, the focus is unchanged.

The following gif demonstrates the new behaviour.

These changes make it much easier to navigate CloudPebble exclusively with the keyboard in that it's an improvement on the current state of tab-focus (when you switch to a new page and then click TAB, you focus on the navigation), but I'm not 100% sure that it's the best solution. Specifically, it might be better to just autofocus on the first form element every time, and not remember the previously focussed element.

[Katharine]

This branch doesn't merge and also doesn't build, due to outdated requirements.

[Spacerat]

I've fixed the conflicts, but we should probably decide on whether this new behaviour is actually an improvement, and if you think it is, it might be worth playing around with it more to iron out any corner-case issues there might be.

[Katharine]

This is why I tried getting it to deploy!