Make i3 compatible with the very latest xcb

This involves:
 • Compiling with xcb-util instead of xcb-{atom,aux} (they merged the libraries)
 • Not using xcb-{event,property} anymore (code removed upstream)
 • Not using the predefined WINDOW, CARDINEL, … atoms (removed upstream)
 • Using the new xcb_icccm_* data types/functions instead of just xcb_*
   (for example xcb_icccm_get_wm_hints instead of xcb_get_wm_hints)

Also I refactored the atoms to use x-macros.

include/all.h

@@ -26,10 +26,14 @@
 
 #include <xcb/xcb.h>
 #include <xcb/xcb_aux.h>
-#include <xcb/xcb_event.h>
 #include <xcb/xcb_keysyms.h>
 #include <xcb/xcb_icccm.h>
 
+/* Contains compatibility definitions for old libxcb versions */
+#ifdef XCB_COMPAT
+#include "xcb_compat.h"
+#endif
+
 #include "util.h"
 #include "ipc.h"
 #include "tree.h"