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
will 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.