For this, we should move the handling of starting and stopping Xmir into libertine-launch and out of ubuntu-app-launch.
Then, ubuntu-app-launch should send SIGTERM to the actual X app pid and wait for the Mir window(s) to be gone. When the windows are gone, but pids still exist, then u-a-l will send SIGKILL to any remaining pids.
For this, we should move the handling of starting and stopping Xmir into libertine-launch and out of ubuntu-app-launch.
Then, ubuntu-app-launch should send SIGTERM to the actual X app pid and wait for the Mir window(s) to be gone. When the windows are gone, but pids still exist, then u-a-l will send SIGKILL to any remaining pids.