Support for adding OPDS catalogs as libraries

[Difegue]

OPDS is an XML-based standard for exposing ebook libraries to readers.
It basically allows a reader to browse an online catalog and download the stuff he wants to read off the server instantly.

It's implemented by a few servers for comics/ebooks, namely Calibre, Ubooquity and LANraragi.

OPDS as a source is implemented by a number of mobile readers, but the desktop offerings are limited to say the least, even more so on Linux/macOS.

Since YACReader has support for multiple libraries already, being able to add OPDS catalogs would be a fitting/convenient feature to avoid duplication of metadata/downloading comics manually.
I realize YACReaderLibrary is already a server for the mobile versions, so this might not interest you at all. 😅

[selmf]

I'm not sure. Our desktop reader app does not have any network capabilities despite version check and the built-in dictionary/translator, so this wouldn't be trivial to implement. The library is a bit better suited to such tasks (I guess), but anything that is passed on to the reader needs to be local, so the reader can pick it up. On desktop, the library does not serve the pages but acts as a fancy launcher with callbacks for the reader. It would be nice if the reader could pick up pages from the library directly or via network, but we're not really there yet and there are other issues that need our attention …

[Difegue]

Oh right, I didn't explain this part correctly: Since OPDS is a standard meant for catalogs, it doesn't provide any form of streaming capabilities: OPDS readers download the file locally before reading it.
Think of it as a form of online catalog giving you the metadata and thumbnails at a glance.

There exists an extension to the standard that allows for page streaming, but it's only implemented by Ubooquity.

[selmf]

Not sure if we have the resources to tackle this now. It might be worth looking into after we have implemented other xml related stuff like #96

@luisangelsm what is your take on this?