Avoid the 3-second 'afterlife' of instances by using
kill (pid, 0) instead of checking the file lock. We
trade pid for lock races, but this seems more reliable.
Also add flatpak_installation_launch_full that returns
a FlatpakInstance for the launched app, and lets us
avoid races around killing the app, by using a child
watch.
Closes: #2221
Approved by: matthiasclasen
Matthias Clasen
6daf195e42