Depend on glib 2.66, use new APIs
This is another larger-than-it-could-have-been MR.
Since GTK depends on glib 2.66, we can safely do that too, as well as use some new APIs that we've been avoiding. One of them is g_assert_finalize_object()
that allows to check memory leaks in tests... And turns out we leak a lot, so fix that. Some of the changes will need to be backported to libhandy too.
Includes !329 (merged)