Improve recording timestamps by adding time zones

[felix11h]

Currently, only naive (time zone unaware) timestamps are used, see also https://github.com/felix11h/sd-copy/blob/master/CONCEPTS.md. This is because in the past, both of my cameras, Fujifilm X-T3 and DJI Osmo Action, did not have the option to set a time zone. With a firmware update to the X-T3, setting time zones is now possible in the Fujifilm camera.

I want to consider adding time zone support. This would allow for setting correct filesystem metadata timestamps (date modified, date created). Currently these are set using the operating system's time zone, making them unreliable.

For cameras or media types without time zone information, the new feature would require setting the time zone explicitly while sorting. A good interface will be needed.

[felix11h]

I'm undecided at the moment whether this feature is worth the additional complexity in the application, and the additional effort when sorting images from sources without time zone support.

It would be nice to have reliable filesystem metadata timestamps, but these can also be modified easily by mistake. It feels like this feature is worth doing only if decide to write the time zone aware timestamps into image or video metadata, which I'm currently decidedly against.