Make all flyout items (with visual representation) focusable

[BenHenning]

Arbitrary flyout items that can be added as of v12 (see #8687 for some context on how this affected the old keyboard navigation plugin).

[BenHenning]

#8939 addresses the key FlyoutButton case, but a longer-term approach may be to change FlyoutItem to force an IFocusableNode as the element. This would be a breaking change unless it were done in a passive "it could be focusable" way.

[gonfunko]

I don't think we can do that because not all flyout items are rendered, and therefore focusable - notably, gaps are FlyoutItems, but they aren't rendered and obviously shouldn't be focusable. They are bounded, however.

[BenHenning]

Hmm. In that case, either we'd need to have a special case version of FlyoutItem or, at minimum, update some documentation somewhere to explain how to make an item properly focusable for keyboard navigation (i.e. hook it up like FlyoutButton). The bigger concern is that "just do what FlyoutButton does" won't actually work as advice, however, since there's a lookup component (which is done in WorkspaceSvg in this case) to actually make it work correctly.

How much are we expecting custom flyout items that are focusable and aren't part of Core? If the answer's more than zero, then we'll need to think carefully about how to make Workspace's find function composable and customizable for external configuration (I have some ideas on how this might work).