Rewritten comment explaining the problem with 'date posted' and UTC

In the end, I don't think there's a much better solution than just to go
for UTC, since many pictures won't have the TimeZoneOffset EXIF tag
anyway, and also because for sorting purposes in the photostream just
interpreting the date as UTC (even if it's wrong) should be enough.

https://bugzilla.gnome.org/show_bug.cgi?id=734193
Reorganized how settings are loaded and stored from/to disk
Move the 'Use date taken as date posted' option from Misc to picture defaults

Make it possible to define a value by default for every picture as is
loaded, as well as to define case by case values if needed.

https://bugzilla.gnome.org/show_bug.cgi?id=734193
Fixed issue setting inconsistent state for toggle buttons in details window