Returns tags through WebDAV

Fixes https://github.com/owncloud/core/issues/12856

I figured out how to make it return a list of tags :smile: 

@DeepDiver1975 @dragotin @icewind1991 

- [x] only select/return tags when explicitly requested
- [x] separate tags from favorite ?
- [x] unit tests
Do you think we'll have cases where getChildren() from a Directory node will not return OC_Connector_Sabre_Node ? I don't think it's necessary. (there's already a check for OC_Connector_Sabre_Directory above, https://github.com/owncloud/core/pull/12865/files#diff-aa826dde9ddbf6ed6621f1b0cdc524b3R198)