The documentation previously implicitly said in a few places that
interfaces are classed, but reading through the implementation of
`GType`, I don’t think they are. If they were, the registration of the
fundamental `G_TYPE_INTERFACE` in `gobject_init()` would specify
`G_TYPE_FLAG_CLASSED`. It only specifies `G_TYPE_FLAG_DERIVABLE`.
I think this makes sense, because you can’t subclass an interface.
Subclassing is a key property of being classed.
Tweak the `GType` tutorial to remove that implicit statement, and expand
the documentation for `G_TYPE_IS_CLASSED`.
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Fixes: #252
783f1b8640