Allow SDLXLIFF tags with dashes in the ID

Once upon a time " - (dash)" were not allowed in the tag IDs of SDLXLIFF files, since the id was itself used in our internal tag ID, separated with dashes from other data. This is since 2017 not the case anymore, the ID is stored in a separate HTML data attribute. Therefore the '-' restriction can be removed from the SDLXLIFF import.