QDoc: Refactor manifest writer

The examples manifest is only generated by the html generator. This should run regardless of generator type. The code for generating manifests should be extracted and turned into a ManifestWriter, similar to the HelpProjectWriter. As explained by treinio there's a conceptual difference between writers and generators in QDoc; a generator generates output from source to a specific output format, while a writer takes the resulting node tree and spits out something extra.