Restore Inserter menu focus by tracking opening node

[aduth]

Related: #578

This pull request seeks to revise the Inserter Menu close-on-escape behavior to focus the original opening element by tracking the node when the opening toggle event occurs. A goal with this change is to eliminate ancestor access to a button's rendered node through its buttonRef prop (removed by the changes here).

Implementation notes:

event.currentTarget could be interchangeable with document.activeElement, but I chose the former since it feels more reliable to the knowledge that the current target is the rendered button we intend to refocus. The downside is that toggle must be used as an event callback (the only current usage).

Testing instructions:

Repeat testing instructions from #578.

[youknowriad]

👍

[youknowriad]

I like this change. The buttonRef prop was a bit weird.

[youknowriad]

I think the cloneMenu prop should receive a nullable selected block (or slug) as a parameter (even if it's not used right now).

[aduth]

I think the cloneMenu prop should receive a nullable selected block (or slug) as a parameter (even if it's not used right now).

So consolidating to onSelect( slug: ?string ) in place of the onInsertBlock and closeMenu combo?

[youknowriad]

Yeah, or onClose( slug: ?string )

[BE-Webdesign]

👍 This is cleaner for now. If we ever want to open the inserter via keyboard shortcut, we will need to revisit this, but for our current needs, this is a better solution.

[aduth]

If we ever want to open the inserter via keyboard shortcut, we will need to revisit this

Sure. The main objective with these changes it to push for components to be self-sufficient and not depend on particular DOM structures of other components. To this point, with keyboard shortcuts, it seems sensible that we'd expose this in a way that the component can define its intent to respond to global keyboard events (perhaps through a higher-order component), rather than have the keyboard event handler call on the inserter(s) directly.

[aduth]

In the rebased c98a7bb, I consolidated further to a single onSelect prop which accepts a (nullable) slug of the block to insert.

I also found that shifting focus has a tricky gotcha which is to be exempt from click outside behavior; clicking a blank spot on the page shouldn't be forcing the button to be focused. This is accounted for in 55829f1 to only react to the escape key press (or, more accurately, the menu closing without a selected slug).