Tags are now handled by a tag manager class which stores sector/tag pairs. This way multiple entries can be added per sector.
Since UDMF does not have any arrays the additional tags are stored as a space separated string as 'MoreIDs'.
This is done to encapsulate the gory details of tag search in one place so that the implementation of multiple tags per sector remains contained to a few isolated spots in the code.
This also moves the special 'tag == 0 -> activate backsector' handling into the iterator class.
