Bump min Qt/KF versions to those of dolphin

Summary:
With Dolphin being the only known user of baloo-widgets currently, having
the same min deps helps applying the same coding standards (e.g. using
nullptr etc).

Reviewers: #baloo, #dolphin, ngraham, elvisangelaccio

Reviewed By: #dolphin, ngraham, elvisangelaccio

Subscribers: ngraham

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D13886