Add suport for <url> tags inside <release>

[jgarciao]

Since appstream 0.12.5 (released in Jan 2019) the <release> tag can have a <url> tag inside

https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-releases

Each release tag may have a description tag as child, containing a brief description of what is new in the release. The description tag is structured as described in .
A release may also have an url tag as child. The release url should point to detailed release notes that explain the changes made in this particular release. The url tag may have a type property with details as the only currently allowed value. If the type is missing, an URL type of details is implicitly assumed.

Some apps like Cheese already use this tag and website shows (null) in the release description: