Implement a metadata system for user-made content.

When loading add-ons (currently only multiplayer scenarios and campaigns), _main.cfg is no longer loaded directly.
Instead a metadata file, _meta.cfg, is automatically generated for each _main.cfg. This file contains all data necessary during the mp_create dialog.
Once a level is selected in mp_create, _main.cfg for that level is loaded.
NOTES:
- Metadata files are kept up-to-date by comparing their dates modified to those of the rest of the files in their respective directories.
- There is an issue of making sure that any necessary defines are loaded before attempting to load _main.cfg in order to generate metadata.
  - i.e. if the data we need is in an #IFDEF.
  - for now, this has been remedied by simply defining MULTIPLAYER, as this is most common. If anyone knows of others, please let me know.
- I may have missed some attributes which should be included in metadata, but these can easily be added to the generating code. Again, please point them out if you see them.
- I have yet to implement metadata for the main data directory. This should hopefully be trivial.
- All preproc defines, including campaign define=, are still functional. Campaign defines in particular, though, (as well as campaign xtra defines) are now unnecessary, as
  this system accomplishes the same goal automatically for all content.