Language switcher widget

Adds a fairly simple WordPress widget offering language switching functionality.

User can choose to show available languages as a `<UL>` list or `<SELECT>` drop-down (using the same js as the core WP archives drop-down).

There's also a checkbox allowing you to show or exclude languages when there's no appropriate equivalent to link to. In list mode, it just adds a `<LI>` with no link. In drop-down mode, the `<option>` is disabled.