Various country flag improvements

* Added country flags for games from Netherlands and Spain
* Added separate category for Region Free games (Uses European flag as placeholder)
* Added missing country filter options in "show regions" menu
* Rearranged country filters for readability
* Incremented CACHE_REVISION

Also fixed various country filters not showing up as options in the "Show regions" menu.
Removed SDK flag

SDK is not an official Nintendo country code, and is not actually used
by any official software.
Converted country flags for DolphinQt