Allow globally overriding slots in the light dom of components and use it to override the auth-info slot in the Nextcloud file picker

In newer versions there is now a normal method for this which also supports
dynamically added components.

The static version is deprecated and removed in 2.0, so we need to port here.

Remove duplicated clipboard warning, disable nextcloudfilepicker upload button if no files are selected

Pull out nextcloudfilepicker styles to file-handling styles, styling, translation and function of clipboard