[Patch] Fix timezones and DST for image correlation

[Bjoeni]

Not sure if it was a defect, but certainly very unexpected behavior for me.

I'd expect the timezones set in the dialog to be absolute, so if the GPS track is ZULU, I'm currently in Germany (UTC+2) and I'm geotagging images taken in Thailand (UTC+7), I should set the timezone to "+7:00" (that's why it's called 'timezone' and not 'offset').

However at the moment it is indeed depending on the timezone you are in, so when I tried to correlate the images from Thailand while being in Germany, I had to use "-5:00" as a timezone, whereas the same task in New Zealand would require the timezone to be "+6:00".
This is because the EXIF usually doesn't contain timezone data and the EXIF reader applies the current timezone, which leads to the same picture once being i.e. 13:00 NZST and once 13:00 CEST (which is obviously very different).

The patch fixes this by completely ignoring the local timezone when reading EXIF data (assuming ZULU) and only using the local timezone as default when prefilling the textbox.
It also allows the user to activate/deactivate daylight saving time when synchronizing from a photo (separate enhancement). This currently leads to an offset of 3600 secs which doesn't really seem to make much sense either.

[Bjoeni]

Just noticed that apparently I didn't upload the latest version of the patch. Uploaded it now as V2.

[Bjoeni]

And btw. this might actually be a regression of [14079]. Before, GPX trackpoints and Photos were both (wrongly) assumed to be in the local timezone, currently GPX is in UTC and Photos are in the local timezone.

[Don-vip]

In 15045/josm:

fix #17576 - Fix timezones and DST for image correlation (patch by Bjoeni, modified)

[Don-vip]

In 15047/josm:

see #17576 - update ExifReaderTest

[Bjoeni]

Ticket #5087 has been marked as a duplicate of this ticket.