Rename acceptsFocus as acceptsActiveTarget

I don't want to conflate the concept of focus and the concept of being
activeTarget.
Add API to Target around becoming activeTarget

In particular I've added a rough menu/toolbar example that uses these
new API hooks to preserve the original activeTarget while allowing
the menuItems themselves to become the activeTarget for handling
keyboard events.

While we may have some opinionated components that do otherwise,
this activeTarget system doesn't limit interesting multi-input
scenarios.