Support file url album art - Desktop

Summary: If the android code sees a file url, and album art is needed, it will start a request to transfer the album art. This code does some sanity checks to prevent abuse and then transfers the album art.

Test Plan: Art is transferred succesfully.

Reviewers: KDE Connect, nicolasfella, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: nicolasfella, albertvaka

Differential Revision: https://phabricator.kde.org/D11017