Menu items translation: Filter to display only multilingual items

First of all, this is a very useful module, thanks for it!

A feature request:
on a multilingual site, most of the menu items have a language set. E.g.

- The menu item My page, set to English, links to a node in English, called My page.
- The menu item Ma page, set to French, links to a node in French, called Ma page, which is the French translation of the node My page.

So there is no need to translate these items.
Only menu items with language set to "All language" may need translation.

It might then be confusing for the end user to be shown all the menu items, even the ones that do not need translation.

The attached patch adds a new filter, checked by default, that allows to display only multilingual menu items.

Please review it :-)

By the way, pager handling might need improvement.