Proper l10n of items per page in the screen options

[SergeyBiryukov]

Sites, Users, Comments, Media items and Plugins have their own translation context on the screen options tab, e.g. “sites per page (screen options)”.

To use the right grammatical case, Posts, Pages, Categories and Tags should have that context too.

Related #13673

Refreshed for 3.3.

Also, for proper translation all of these strings should support single and plural forms.

That part won't be implemented, according to nacin's comment on #13673.

The context, however, is still needed, so this is what the patch does.

15576.2.patch​ isn't very backwards compatible. per_page should be implemented like menu_name was -- if it isn't set, it goes forward with the existing "name" argument, rather than defaulting to "Posts"/"Pages".

Replying to nacin:

per_page should be implemented like menu_name was -- if it isn't set, it goes forward with the existing "name" argument, rather than defaulting to "Posts"/"Pages".

Thanks, done in 15576.3.patch​.

As an alternative, we could replace "Posts"/"Pages" with "items" string (15576.4.patch​), which we already have in options-reading.php:
http://core.trac.wordpress.org/browser/tags/3.4.1/wp-admin/options-reading.php#L111

The use case is essentially the same, so the context might not be needed. 15576.5.patch​ adds it just in case (and uses the uppercased word for consistency with other screens).

With 15576.6.patch​, we could remove 9 strings by making the same change on the other screens (for consistency as well).

Moving to "items" across the board is a decent idea. We can talk about it in 3.6.

To summarize, "20 Записи" (20 Posts) isn't a valid expression in Russian.

It should be "20 записей", but it's currently impossible to use the proper translation due to the lack of context. I imagine Czech (according to pavelevap's comments on #13673) and other languages with declensions have the same problem here.

Introducing a new post type label just for screen options seems like an overkill, hence the "Items" suggestion.