[baloosearch] Allow specifying a time when using e.g. mtime

Summary:
Currently, the search results can only limited by date, e.g.
baloosearch 'mtime>="2019-04-06"', but sometimes limiting the range
further is nice. Allow specifying a QDateTime in ISO format, e.g.
baloosearch 'mtime>="2019-04-06T22:10:20"'.

Test Plan: see commands in summary

Reviewers: Baloo, Frameworks, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20327