Renaming GameData directory to Metadata for clarity.
Localizing savegame string constants

Removed a global constants file that held all of the savegame-related
JSON attribute and element identifiers. It created unnecessary
compilation dependencies: changing a single string would require a lot
of unrelated files to be recompiled unnecessarily.

Most of the string constants were only ever used by one class each, and
so they could easily be moved to these classes.