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>