Misc: Code cleanup & debug code changes

- Merge @daBrado's focus event debugging code. Thanks!

- Use macro to reduce code redundancy in various functions.

- Move focus event validation from ev_focus_out() to a separate
  function.

- Add logic in ev_handle() to increase the chance of successful window
  name detection if compton is not reading window names normally (i.e.
  if there's no --shadow-exclude), when DEBUG_EVENTS is on.

src/compton.h

@@ -1009,7 +1009,7 @@ win_get_class(Display *dpy, win *w);
 static int
 ev_serial(XEvent *ev);
 
-static char *
+static const char *
 ev_name(XEvent *ev);
 
 static Window