Add fyne repository implementation for WebDav

[brucealthompson]

Checklist

• [X] I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• [X] This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

WebDav is a common tool used for browsing remote filesystems. It is fully supported in Golang. This feature request is to support WebDav using the fyne repository interface

Is it possible to construct a solution with the existing API?

Yes. Its just a new instance of the existing repository interface.

Describe the solution you'd like to see.

A new repository interface for the webdav schema following RFC 4918