This fixes several issues related to parsing of parent PIDs:
* A process with lines or spaces or parentheses in its process name
would confuse the code in X.A.SpawnOn and possibly lead to a
`Prelude.read: no parse` exception.
* `X.H.WindowSwallowing.isChildOf` looked for the parent PID anywhere in
the output of pstree, so single-digit parent PIDs would be considered
as parents of any process with that digit anywhere in its chain of
parent PIDs. (Note that apps in PID namespaces like in Flatpak often
have single-digit PIDs.)
* `pstree` is no longer required in `$PATH`.
Fixes: https://github.com/xmonad/xmonad-contrib/issues/726
