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