Small refactoring of InfoExt::ISignalInfo
Introduce ReturnValueInfo
Move skip flag into ReturnValueInfo
Rename SetterArgumentInfo to FieldArgumentInfo
Add #direction to argument info classes
Remove superfluous constructor override
Add #name to argument info classes
Make BaseArgumentBuilder constructor take an ArgumentInfo
Move storage of argument info into BaseArgumentBuilder
Extract methods direction, name and type_info in BaseArgumentBuilder
Introduce factory methods for MappingMethodBuilder
Move builder creation from initializer to factory methods
Move argument builder relation setup to factory methods
Remove unused MappingMethodBuilder#argument_infos
Pass receiver and user data infos to MappingMethodBuilder#for_signal separately
Create and handle builders for receiver and user data separately
Remove superfluous subclasses of ReturnValueInfo
Handle array length parameters in MappingMethodBuilder
Remove overrides in CallbackArgumentBuilder for #post end #retval

Due to the override of needs_outgoing_parameter_conversion?, these overrides are
no longer needed.
Simplify ReturnValueBuilder#retval