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.
The original plan was to allow each system to have customizable lists of
metadata, which made constructing metadata really painful (the declaration
list isn't stored on the metadata instance because that's pretty wasteful
for 2,000 games). Now they're constructed by passing a MetaDataListType
enum in the constructor.
Declaration lists are now managed by const globals passed by reference
through getMDDByType(MetaDataListType).
Added GuiMsgBoxOk and GuiMsgBoxYesNo, basic message boxes.
Added rating scraping to TheGamesDB scraper.
Added warning if platform ID is not set for a system the user has selected
to scrape.
Scraper now contains a partial implementation since almost all scrapers
will follow the pattern of create HttpReq -> create and wait for
AsyncReqComponent -> parse HttpReq contents.
