COMMON: Rewrite Encoder and drop dependency on iconv (#2586)

Different platforms have different levels of support of encodings and
often have slight variations. We already have tables for most encoding
with only CJK missing. Full transcoding inclusion allows us to get reliable
encoding results independently of platform. The biggest con is the need for
external tables encoding.dat.

It removes a duplicate table for korean in graphics/korfont.cpp

backends/platform/sdl/sdl.h

@@ -169,8 +169,6 @@ protected:
 	virtual bool setGraphicsMode(int mode, uint flags) override;
 	virtual int getGraphicsMode() const override;
 #endif
-protected:
-	virtual char *convertEncoding(const char *to, const char *from, const char *string, size_t length) override;
 };
 
 #endif