Add save file type inside translation string

File type (Matroska, OGG, MP4) now is included into the
translation string as a placeholder, like in '%1 files'.
This way, translations can be more flexible. Languages with
a grammar where 'files' cannot be kept behind the save file
type can now make use of the placeholder to put the file
type anywhere they need. Example, Spanish: 'MP4 archivos' now
can be translated as 'Archivos MP4'.
Merge pull request #113 from danirod/glinject-next

Add save file type inside translation string.