Add support for g_autoptr() to our types

g_autoptr() is a macro that was defined in GLib 2.44 that allows for basic auto-cleanup of variables. One way to add this kind of support would be through the use of e.g. G_DECLARE_DERIVABLE_TYPE() for our declarations, but this would consitute an ABI break (due to the ...Private * field in the public structs). Instead, we can use G_DEFINE_AUTOPTR_CLEANUP_FUNC to manually declare this.

This commit also bumps the minimally required GLib version to 2.44