Write unit test case for the Qt-Bridge asset importer

Before adding Sketch importer write a good unit test for the existing features. We don't want to break the PS import while we add the Sketch specialisation.

1. Test the Model infrastructure sanity. Done
2. Test Node Parsers.
3. Test Metadata parsers.
   1. Basic. Done
   2. Text. Done
   3. Including external and internal references.
   4. Including aliases.
4. Test parsing of Custom QML Properties. Done