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.)

src/config.c

@@ -11,9 +11,7 @@
  *
  */
 #include "all.h"
-
-/* We need Xlib for XStringToKeysym */
-#include <X11/Xlib.h>
+#include <xkbcommon/xkbcommon.h>
 
 char *current_configpath = NULL;
 Config config;