Change some RString methods to free functions

These ones aren't a std::string method. Doing this helps the RString to
std::string migration.

src/FontCharAliases.cpp

@@ -364,7 +364,7 @@ static void InitCharAliases()
 	{
 		RString from = i->first;
 		RString to = WcharToUTF8(i->second);
-		from.MakeLower();
+		MakeLower(from);
 		CharAliasRepl[from] = to;
 	}
 }