Add test for CollectionScheduler, code cleanups, bugfixes.

Summary:
Wrote a unittest just for fun. Found lots of interesting things...

* Methods called in the wrong thread, leading to QTimer warnings
* Unscheduling all collections with the same "next time" when one collection is removed
* for() loops with if+return, where find_if() is much more readable
* Unnecessary detaching (non-const lowerBound)
* All this will break in 2038 (32 bit time in seconds), will fix in next commit

Test Plan: bin/collectionschedulertest

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20362