New option in 'Misc' tab in settings: 'Use date taken as date posted'

https://bugzilla.gnome.org/show_bug.cgi?id=734193
Load DateTimeOriginal for 'date taken' from EXIF information

EXIF's DateTime changes whenever the file changes (e.g. edited) so it's
not a good value to use for the 'date taken' field. DateTimeOriginal
actually represents the moment the image was capture, so use that one.

https://bugzilla.gnome.org/show_bug.cgi?id=734193
Updated FrogrController to allow setting 'date taken' as 'date posted'

This is not 100% correct yet, because the current implementation assumes
that date and time extracted from the EXIF information is UTC, but that
does not have necessarily the case.

https://bugzilla.gnome.org/show_bug.cgi?id=734193