glib: Deprecate PopplerDocument date properties

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.

Fixes #765
6 jobs for GDateTime in 8 minutes and 45 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1314175
build

00:03:12

passed #1314180
build_android

00:02:47

passed #1314176
build_clang_libcpp

00:08:45

passed #1314179
build_clazy

00:02:34

passed #1314178
build_mingw64_fedora

00:02:00

passed #1314177
build_ubuntu_16_04

00:02:00