GObject: fix property override type checks

The property override typecheck was meant to enforce the type on the
overriding property being exactly equal to the type on the interface
property.  Instead, g_type_is_a() was incorrectly used.

We could try to enforce equality, but if a property is read-only then it
should be possible for the implementation to type the property with any
subtype of the type specified on the interface (because returning a more
specific type will still satisfy the interface).  Likewise, if the
property is write-only then it should be possible for the implementation
to type the property with any supertype.

We implement the check this way.

https://bugzilla.gnome.org/show_bug.cgi?id=666616