Provide WebDAV functions

[susnux]

Currently there are a couple of places where we need the same functions, like checking if a path exists, get the directory content, query favorite files, etc.

Examples are the files app and the file picker.

So I think it would be useful to have those functions at one loction, which could be this package.
We could provide some API like:

/** Get Node for a path or undefined if it does not exist */
getNode(path: string): ?Node
/** Get directory content */
getDirectoryContent(node: Node): Node[]
/** Get favorite nodes (files + directories) */
getFavorites(): Node[]
/** Recently changes files */
getRecentlyChanged(): Node[]

(of cause probably other parameters are required, but just to get an idea what I mean.

cc @skjnldsv