XML: Move version string into attribute

[FreezyLemon]

Closes #842.

This PR changes the read and write functions for data/db/anime.xml, data/user/<user@service>/anime.xml and data/user/<user@service>/history.xml.

It does not change /data/feed/history.xml (does not contain a version string) or /data/settings.xml (see #842, there is a meta element, but it is inside another element so the XML is already valid)

Related to XmlReadAttr: Calling .value() on an attribute that might not exist is allowed as per documentation:

In case the attribute handle is null, both functions return empty strings - they never return null pointers.