shell: Make opening file year-2038-safe

Replace GTimeval-related API by the year-2038-safe GDateTime when
opening files.
4 jobs for wip/gpoo/fix-ev-window-deprecations in 7 minutes and 53 seconds
Status Job ID Name Coverage
  Build
passed #794615
flatpak
flatpak

00:07:53

manual #794612
allowed to fail manual
master
passed #794613
meson

00:00:43

manual #794614
allowed to fail manual
meson-gtk-doc