Add a simplified backport of g_steal_pointer()

Add a simplified backport of g_steal_pointer()

This will be used in tests later in the branch.

Sadly we can't use GLIB_VERSION_2_44 unless we are willing to have a
hard dependency on GLib 2.44, which would force us to do all our
Travis-CI builds in Docker containers rather than in ye olde base
system, and that adds 50% to the time taken to do builds.

Reviewed-by: Philip Withnall <withnall@endlessm.com>
[smcv: Rebase onto 1.13.x branch, fix minor conflicts]
Signed-off-by: Simon McVittie <smcv@collabora.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=101354
(cherry picked from commit d5742550ca566317eaddea0ff7db04098f9f044f)