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.

common.mk

@@ -40,8 +40,13 @@ LDFLAGS += -lm
 LDFLAGS += -lxcb-event
 LDFLAGS += -lxcb-property
 LDFLAGS += -lxcb-keysyms
+ifeq ($(shell pkg-config --exists xcb-util || echo 1),1)
+CFLAGS += -DXCB_COMPAT
 LDFLAGS += -lxcb-atom
 LDFLAGS += -lxcb-aux
+else
+LDFLAGS += -lxcb-util
+endif
 LDFLAGS += -lxcb-icccm
 LDFLAGS += -lxcb-xinerama
 LDFLAGS += -lxcb-randr