qdoc: Keep track of attributes written to example manifest files

The content written to example manifest files can be extended with
additional metadata in the form of xml attributes. In order to
avoid invalid (duplicated) input from generating malformed xml,
QDoc now keeps track of the attributes and only writes metadata
that hasn't already been written.

Task-number: QTBUG-46692
Change-Id: I645935fa8f32b915b7335c400f5a8f2cf72802b7
Reviewed-by: Martin Smith <martin.smith@digia.com>