Fixing l10n master

Mainly moving around files between lib/l10n and lib/private/l10n

In addition the extraction script can now handle translator comments.
Whenever a comment from the source code shall be move over to the translations just use the tag TRANSLATORS.

e.g.
````
// TRANSLATORS: %n is the number of minutes
$l->t('%n minutes ago');
````


@karlitschek @blizzz @bantu @icewind1991 @bartv2 @schiesbn THX