Returns and update tags through WebDAV PROPFIND and PROPPATCH

Added oc:tags and oc:favorites in PROPFIND response.
It is possible to update them with PROPPATCH.
These properties are optional which means they need to be requested
explicitly
Remove obsolete method
Fix code style issues for tags plugin
Merge pull request #12865 from owncloud/files-tags-webdav

Returns tags through WebDAV