Cache games without banners

Games without banners were not cached before, because a banner could
become available at any time, making the cache outdated without it
becoming invalidated. Instead of not caching anything, this change makes
Dolphin check for a banner every time a cache that lacks a banner is read.
This is faster than reading all metadata, because reading a Wii banner
only reads from the game's save file, not the volume and its filesystem.

The cache revision is incremented, because otherwise banners will be
missing if a cache without a banner is created in the new version and
the user switches to an old version and creates a savefile.

Source/Core/DolphinWX/ISOFile.h

@@ -81,4 +81,6 @@ private:
 	void SaveToCache();
 
 	std::string CreateCacheFilename();
+
+	void ReadBanner(const DiscIO::IVolume& volume);
 };