Expose permission via WebDAV

- Introduce isShared() and isMounted() on FileInfo class
- Reuse these methods on determineIcon()
- Generate permission string for the desktop client
- expose {http://owncloud.org/ns}perm as additional WebDAV property containing the permission string

FYI:
@danimo @dragotin @icewind1991 @karlitschek 

I will work on the detailed list of permission with @dragotin next week

refs https://github.com/owncloud/core/issues/8322