QDoc: don't write default values for attributes to index files

In conversation with treinio, the suggestion to drop the myriad of attributes that QDoc write to index files that have no effect at all, because their value is the default anyway when reading things back in. As an example, all non-virtual non-const non-static/final/overridden functions gets the attributes

virtual="non" const="false" static="false" final="false" override="false"

QDoc should treat 'false' as default for those and only write the attributes when they're true. This would likely save a noticeable bit of the typical index file size and possibly speed things up a bit.