keybindings: Keep keybindings in an hash table instead of an array

This allows us to look for a match with an O(1) search instead of O(n)
which is nice, particularly when running as a wayland compositor in
which case we have to do this search for every key press event (as
opposed to only when our passive grab triggers in the X compositor
case).

We actually need two hash tables. On one we keep all the keybindings
themselves which allows us to add external grabs without constantly
re-allocating the array we were using previously.

The other hash table is an index of the keybindings in the first table
by their keycodes and mask which is how we actually match the key
press events. This second table thus needs to be rebuilt when the
keymap changes since keycodes have to be resolved then but since we're
only keeping pointers to the first table it's a fast operation.

https://bugzilla.gnome.org/show_bug.cgi?id=725588

src/core/display-private.h

@@ -232,8 +232,8 @@ struct _MetaDisplay
   int	      grab_resize_timeout_id;
 
   /* Keybindings stuff */
-  MetaKeyBinding *key_bindings;
-  int             n_key_bindings;
+  GHashTable     *key_bindings;
+  GHashTable     *key_bindings_index;
   int             min_keycode;
   int             max_keycode;
   KeySym *keymap;