backports: Backport new NONE/DEFAULT constants from GLib 2.74

These enums were not originally defined with a zero-valued constant (or in the case of GApplicationFlags, the constant always existed but its name was inappropriate for GObject-Introspection), and the corresponding constants were added in GLib 2.74 to make them more self-documenting.

Based on !37 (merged) (so please review that first), and should not be merged until the API of GLib 2.74.0 has been frozen (which has now happened).