This was to resolve some circular dependencies with the portal code.
The most notable changees:
* FTextureID was moved from textures.h to doomtype.h because it is frequently needed in files that don't want to do anything with actual textures.
* split off the parts from p_maputl into a separate header.
* consolidated all blockmap related data into p_blockmap.h
* split off the polyobject parts into po_man.h
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.
