Enhancement: Shares from propfind

Added a new property to the propfind, so that, we can get if the files in a folder are shared directly with just one request.
Previously, a propfind and another additional request were needed to the shares api to retrieve the shares of the folder.

https://github.com/owncloud/android/issues/3711
https://github.com/owncloud/android-library/pull/496
