Move parts of MappingCommon out of DolphinQt

Some of the functions in MappingCommon would be useful to use on
mobile in the future.

Source/Core/Common/StringUtil.h

@@ -235,6 +235,14 @@ inline bool IsPrintableCharacter(char c)
   return std::isprint(c, std::locale::classic());
 }
 
+/// Returns whether a character is a letter, i.e. whether 'a' <= c <= 'z' || 'A' <= c <= 'Z'
+/// is true. Use this instead of calling std::isalpha directly to ensure
+/// the C locale is being used and to avoid possibly undefined behaviour.
+inline bool IsAlpha(char c)
+{
+  return std::isalpha(c, std::locale::classic());
+}
+
 #ifdef _WIN32
 std::vector<std::string> CommandLineToUtf8Argv(const wchar_t* command_line);
 #endif