[UserMetaData] Shortcut attribute queries for the common case
Summary:
Optimize the common case of only a few or no attributes set, no functional
changes.
Bump KF5_MIN_VERSION to 5.60 for KFM:UMD:queryAttributes().
Test Plan:
ctest
Depends on D20967
Reviewers: Baloo, Frameworks, ngraham, astippich, lbeltrame
Reviewed By: lbeltrame
Tags: Baloo
Differential Revision: https://phabricator.kde.org/D21004