Refactor ConfigFile

[madmiraal]

This PR refactors [ConfigFile](https://docs.rebeltoolbox.com/en/latest/api/class_configfile.html) which includes:

• Addressing four memory leaks:
  When loading or saving encrypted files, the _internal_save() and _internal_load() methods do not delete the FileAccess object f.

  RebelEngine/core/io/config_file.cpp

  Lines 179 to 184 in 35a733d

  	if (err) {
  	memdelete(fae);
  	memdelete(f);
  	return err;
  	}
  	return _internal_save(fae);

• Making methods that create heap objects responsible for deleting them.
• Making the save methods const.
• Removing duplicate code.
• Hiding implementation details by moving private methods to an anonymous namespace in the definition file.
• Minimising the size of the header file by using forward declares instead of includes.

This should make ConfigFile easier to troubleshoot and maintain, while applying good coding practices and improving compile times.