keybindings: Do a breadth first search in our keysym to keycode code

Commit 1af0033368 made a subtle change
regarding how XKeysymToKeycode behaves. It does a depth first search
while XKeysymToKeycode is documented to do a breadth first search:

"this function looks in each column of the core keyboard mapping in
turn and returns the lowest numbered key that matches in the lowest
numbered group" - from the XKB library documentation

Looping over all keycodes for each layout and level index makes us go
back to the previous behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=737134

src/core/keybindings-private.h

@@ -101,6 +101,8 @@ typedef struct
   MetaKeyCombo *iso_next_group_combos;
   int n_iso_next_group_combos;
 
+  xkb_level_index_t keymap_num_levels;
+
   /* Alt+click button grabs */
   ClutterModifierType window_grab_modifiers;
 } MetaKeyBindingManager;