keybindings: Make MetaKeyPref, MetaKeyCombo and MetaKeyHandler private

There's no need for these to be public and keeping them all together
in the same header makes it easier when reading the code.

https://bugzilla.gnome.org/show_bug.cgi?id=725588
keybindings: Rename MetaKeyPref.bindings to MetaKeyPref.combos

Let's call it what it really is, otherwise it's just confusing since
MetaKeyBinding also exists.

https://bugzilla.gnome.org/show_bug.cgi?id=725588
keybindings: Use display_get_keybinding() instead of looping explicitly

Instead of looping over an array of keybindings to find the correct
binding, just use display_get_keybinding().

In the next commit, we'll change the array to be a hash map, so this
helps the patch be cleaner.

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