Browser plugin: sort by type first, then display path.

The SabreDAV browser plugin previously displayed all entries in no
particular order. To make the list easier to digest for the end user,
sort all nodes based on their type first, and their display path second.
Use a custom sort order for the node type, that for instance sorts all
directories above all files.
`compareNodes` is non static, there better use `array($this, 'compareNodes`)` instead as a callable