Basic in-app browsing / "open with" for WebDAV mounts

[rfc2822]

Currently, a third-party file manager or the "Share" workaround have to be used to access files on a WebDAV mount.

However, often such file managers are not available / working. In this case, we could at least provide a basic UI that allows to browse through the files and let users open files with other apps.

It's not the goal provide a full file manager, but only a basic possiblity to use WebDAV when there's no file manager.