Control: dwm automation using X properties.

Calling "dwm -x <cmd>" will set the _DWM_EXECUTE property of the root window and
immediately return. The running dwm will be notified of the property change and
parse the _DWM_EXECUTE value into a command name (everything up to the first ' '
or '\0' char) and the rest. The name and the rest will then be passed to the
execute(cmd, args) function, which is expected to be implemented as part of
config.h. An example is provided in config.def.h.