Change some things in these "app is alive" checks:
- The dialog timeout is separated from the ping timeout, in order
to show it again at a constant rate after dismissing, despite in
flight pings. It still shows immediately after the first failed
ping.
- As we want to tap further into is-alive logic, MetaWindow now
made it a property, that other places in code can fetch and
subscribe.
- Motion events trigger ping (as long as there was none other in
flight for the same window), and are counted between ping and
pong, in order to preemptively declare the window as not alive
before there is trouble with event queues being overflown.
This results in a separate logic between "the application does
not respond" and "we are showing the close dialog" so that the
former may get triggered independently.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2122>
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