Daemon mode

On my system, the popup takes ~500ms to launch, possibly in large part due to #183. Other projects which need to launch a window quickly often use a systemd service or shared instance to mitigate the vast majority of the startup time ([kitty](https://sw.kovidgoyal.net/kitty/invocation/#cmdoption-kitty-single-instance), [foot](https://codeberg.org/dnkl/foot#server-daemon-mode), [ulauncher](https://github.com/Ulauncher/Ulauncher/blob/v5/bin/ulauncher-toggle#L3)). It also offers a simpler solution for singleton #179 without file locking, although file locks are still beneficial since they work without the daemon. I'd be happy to send a PR for this, what do you think?