[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