Currently, we have all the plumbing in place so that GTK consumes the
startup notification ID when focusing a window through the xdg-activation
protocol.

This however misses the case that a window might be requested to be
focused with no startup ID (i.e. via interaction with the application,
not through GApplication or other application launching logic).

In this case, we let the application create a token that will be
consumed by itself. The serial used is that from the last
interaction, so the compositor will still be able to do focus prevention
logic if it applies.

Since we already do have a last serial at hand, prefer xdg-activation
all the way over the now stale gtk-shell focusing support. The timestamp
argument becomes unused, but that is a weak argument to prefer the
private protocol over the standard one. The gtk-shell protocol support
is so far left for interaction with older Mutter.