Allow to filter listings by DApp

[franckc]

It would be useful to provide a way for a DApp to only show listings that were created from it. Anecdotally, we just had a developer asking for this functionality.

A possible simple option to start with would be to allow the DApp to store in the listing data a "DApp name" that would identify from which DApp the listing was created from. The Listings could get filtered by "DApp name". Note that this would not prevent the possibility that another DApp or someone writing directly to blockchain/IPFS would use the same DApp name...