Split the focus tracking into a separate
GtkEventControllerFocus, and change the API one more time.
We are back to having ::focus-in and ::focus-out signals.
Update all users.
Add fields for direct descendents to GtkCrossingData,
and populate them when emitting focus change events.
Also add accessors for these fields to GtkEventControllerKey,
and verify that they are set properly in the focus test.
Not done yet: Do the same for pointer crossing events.
This event controller is meant to replace usage from key-press/release-event
handlers all through. Optionally it can be set a GtkIMContext, so interaction
is carried by the controller.
