D20967: [UserMetaData] Add method to query which attributes are set

[Stefan Brüns]

bruns added subscribers: Windows, FreeBSD.

REPOSITORY
  R286 KFileMetaData

REVISION DETAIL
  https://phabricator.kde.org/D20967

To: bruns, #baloo, #frameworks, astippich, ngraham
Cc: #freebsd, #windows, kde-frameworks-devel, LeGast00n, domson, ashaposhnikov, 
michaelh, astippich, spoorun, ngraham, bruns, abrahams