fix issue with gt.ngettext and variable as third argument

when (what is most likely in all projects the case) gt.ngettext is
called with a variable instead of a literal as the third argument (the
argument that is used to determine wether to use plural or singular
form), it hadn't been detected as a gt call.

This commit also adds a failing spec for this issue that works with the
patch.
hide warning about unknown module for filtered items

only show the warning about items without module information if item
will not be filtered out of the catalog anyway.