Fixes #790: InheritedFrom is no longer in XML Structure

The inherited_from pseudo-tag is no longer inserted in the structure.xml XML
AST file. This commit restores that and adds a test for it.
Merge branch 'develop' into feature/790

Conflicts:
	tests/unit/phpDocumentor/Plugin/Core/Transformer/Writer/XmlTest.php
Merge pull request #1260 from mvriel/feature/790

Fixes #790: InheritedFrom is no longer in XML Structure