Improve skip reason
Make ISignalInfo test run stand-alone
Update skip messages
Support definition of VFuncs returning enums
Add a FIXME regarding mixup of callbacks and vfuncs
Implement support for vfuncs returning enums
Refactor UserDefinedBuilder#type_info
Refactor UserDefinedBuilder#setup_properties
Refactor complicated #getter_def and #setter_def of PropertyBuilder
Move property accessor setup to PropertyBuilder
Inline method WithLayout#setup_accessors_for_field_info
Extract method UnintrospectableTypeInfo#parent_gtype