Optional argument never provided. Cleanup.

Summary:
Let QDateTime convert to local time

Fix Allow already open devices for parsing devices

Fix porting bug. fromSecsSinceEpoch is a static method

Add a backup start/end time, and don't use strings as transport for some dates

Add a couple of alternative keys

0x0023 is 0 and not very unique.

Reviewers: mlaurent, dvratil, vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D14885