Support X11 idle monitor again
Several distros include GNOME Break Timer as a package which does not install in a sandbox, so users may (reasonably) expect to use it in arbitrary X11 sessions. Break Timer used to support this with an idle monitor backend based on XRecord. It's a little late, but if it isn't trouble it would still be nice to bring this back. It doesn't need to work in a Flatpak, so it can be a build option with a feature flag based on the presence of libxtst.
This will solve issue #9 (closed).