This works around the need to take a custom mutex twice and add the
object to a GSList of objects that are currently in construction for the
common case. Only when the constructor is overwritten do we use the
previous behavior and allow things like singleton objects.
The only slightly incompatible change is that previously, it was ok to
call g_object_set() on construct-only properties while the object was
initialized. This will now fail. If that behavior is needed, setting a
custom constructor that just chains up will reenable this functionality.
https://bugzilla.gnome.org/show_bug.cgi?id=557151
2a78adc5e3