Support is added for setting _NET_WM_PID automatically. This is to
support scripting of the window environment. For example, this makes it
possible to script opening a window with same CWD:
1. Retrieve the current window
2. (new) get PID of window
3. Check if it's Alacritty, find first child (presumably a shell), and
get the child's cwd.
4. Spawn new instance of terminal with cwd.
Unaddressed in this commit is how this will coexist on a Wayland system.