wayland: Add windows when readyForPainting changes
A window is added to the workspace when it's mapped. It's assumed that the first Window::windowShown signal indicates that. But it's not entirely true. For example, if setHidden(false); setHidden(true); are called in succession, the window will be marked as ready for painting even though it isn't.
The Window::readyForPaintingChanged() signal fixes that. It's emitted when the window is actually mapped.