You don't need to dynamic_cast everywhere to check things anymore.
Folders can have metadata now (currently not set up).
Metadata is now a public member variable instead of a function that
returns a pointer to make actually using const possible.
Also add a "rating" and "timePlayed" variable to GameData. Some cleanup
in GameData and FolderData. Added sorting functions for rating and
timesPlayed to FolderData. Testing and UI support still tbd.