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.
Merge branch 'control'

Conflicts:
	dwm.c
Control: automation facilities: search/activate.

Similar to some of the functionality provided by wmctrl/xdotool via EWMH:

- activateclient(c): focus c if it's visible, focus and switch to first
  tag in c->tags if it's not currently visible.

- searchclient(pattern, onlyvis): returns the first client c for which the given
  pattern (a POSIX regex) matches the string "<title>###<class>", where <title>
  = c->title and <class> = c's class hint. Return NULL if no client matches the
  pattern. If onlyvis is true only the currently visible clients are considered.

An example is provided in config.def.h implementing the command

  activate <onlyvis> <pattern>
Merge branch 'control'

Conflicts:
	dwm.c