Improve identification of OS-modified tags

[BinToss]

As implied in an Invader discussion.

Ideas:

• new file extension (.YTag? e.g. ".gbxmodel.ytag")
• move OS-specific data outside the stock structs wherever possible. Pointers may be useful in this endeavor.
• Split a tag into two parts/files:
  1. Standard HEK structs
  2. Yelo structs
     • The Standard tag could be used alone, but the Yelo tag-part will be automatically loaded into the scenario if it is found in the tags directory. Add a way to block/remove the Yelo tag from the scenario before compiling.

Remember to support legacy-OS tag structs as well.