Improve C module mocking and drawin signals.

Handle valid as a read-only property in fake capi.

Add the deprecated capi alias data -> _private.

Small fixes.

Forward object signals to their class module; Move fake capi into a module.

Testing dependency.

Unify offscreen (based on Gdk Offscreen window instead of GtkOffscreenWindow) and composition into one widget.

Use composition for direct gtk widget.

Nits.

Spawn test.

Handle any startup notification at the beginning.

Initial commit.