glib: Deprecate GTime fields and properties
GTime was deprecated in glib 2.62 and will overflow in 2038.
The PopplerAttachment struct publicly uses GTime for the 'mtime' and 'ctime' fields. Deprecate these two fields and add accessor functions, poppler_attachment_get_ctime and poppler_attachment_get_mtime, which retun GDateTime* instead.
PopplerDocument's creation-date and mod-date properties are 32-bit unix times, a.k.a. GTime, and will overflow in 2038. Deprecate these properties and replace with creation-datetime and mod-datetime, which are GDateTime instead.