Backport !723 “Fix ref/locking behaviour of gs_app_get_version_history()” to gnome-40

Trivial backport of !723 (merged) to gnome-40. Note that this breaks the libgnomesoftware API by changing the refcounting of the return value of gs_app_get_version_history() (and the null handling, although the behaviour has become more permissive, so that wouldn’t break any app). I don’t see another way to fix this bug without breaking API.

This is not an ABI break, as it’s only the refcount of the return value which has changed. In the worst case, existing external plugins will start leaking the return value if they are not adjusted to the new API.

Signed-off-by: Philip Withnall pwithnall@endlessos.org