Return type overrides

[jwharm]

In a few situations, we need to override the return type of a method:

• When a method in a subclass overrides a method in a superclass, with a different return type
• When an instance method and virtual method in the same class and the same name have differnt return types.

In previous Java-GI versions, the return type would be updated with a patch, but that would also impact the actual call to the native function. That can lead to segfaults. So with this PR, the return type of the Java bindings is changed, but the native function invocation still uses the type signature as declared in the GIR file.

The following return types are changed:

• void GtkMediaStream.play() returns void, but a virtual method with the same name returns gboolean. The instance method now also returns gboolean (always true).
• void GstAudioSink.stop() overrides gboolean GstBaseSink.stop(), so it now returns gboolean (always true).
• void GstVideoOverlay.setRenderRectangle() returns void, but a virtual method with the same name returns gboolean. The instance method now also returns gboolean (always true).
• void SoupAuthDomain.challenge() returns void, but a virtual method with the same name returns char*. The instance method now also returns char* (always null).