Switch to xcb-xkb and libxkbcommon

This removes our last dependency on Xlib! :)

(Okay, an Xlib dependency still comes in through other libraries that we
 link against, but it’s not us. Our code is simpler by this change and
 uses one less connection to X11.)

include/handlers.h

@@ -13,6 +13,7 @@
 #include <xcb/randr.h>
 
 extern int randr_base;
+extern int xkb_base;
 
 /**
  * Adds the given sequence to the list of events which are ignored.