keybindings: Resolve on us layout too if primary is not latin based

If a non-latin based keyboard layout is active, for example Cyrillic,
keybindings won't work unless we resolve the bound keysyms on a
secondary latin based layout. So, to make keybindings work on non-latin
based layouts, detect if a keymap doesn't have all of the basic latin
letters (a-z) and resolve from an additional US layout as well.

https://bugzilla.gnome.org/show_bug.cgi?id=787016

src/core/keybindings-private.h

@@ -116,8 +116,8 @@ typedef struct
   int n_iso_next_group_combos;
 
   /*
-   * A primary layout, and an optional secondary layout that is
-   * not yet used.
+   * A primary layout, and an optional secondary layout for when the
+   * primary layout does not use the latin alphabet.
    */
   MetaKeyBindingKeyboardLayout active_layouts[2];