This commit adds a way to leverage the xproperty and startup_id APIs
to persist an execution token across restarts. It allows to use
`awful.rules` on clients that were executed by a previous Awesome
instance.
The main limitations of these methods is the lack of entropy used to
build the token. If the command is the same in multiple
`awful.spawn.once`, then it will not work as expected. To mitigate this
issue, the system try to concatenate the `awful.rules` table after the
command and hash the resulting string. Given rules are a table, it can
have loops and/or issues with keys ordering. The hash function sort and
limite recursion to prevent a stack overflow. Another issue is the
unreliability of startup notifications.
927e7b2796