Ability to set a preferred display order for switcher links

Current display order (using `bbl_get_switcher_links`) is alphabetical based on language code. Which is a good, neutral starting point. But you can bet people will want it to sort in a particular order.

WPML offers drag-and-drop ordering:
http://wpml.org/documentation/getting-started-guide/language-setup/language-switcher-options/


Maybe make the rows of the 'Available Languages' table sortable, à la 10up's Simple Page Ordering plugin:
http://wordpress.org/plugins/simple-page-ordering/

Start by reading the bbl_get_switcher_links entry point and locating the “Available Languages” table implementation. Determine how a saved language order would affect the switcher output, then verify that the configured order is preserved when links are displayed.

Written by the indexing model from the issue text.