Provide stub methods for features that are irrelevant for internal and popup windows

I did a similar change long time ago, but it was negatively received. Even with that negative feedback, I still would like to push for stubbing methods that are relevant only to toplevel-like windows.

After introducing the WaylandClient class, adding support for new shell surface protocols is now a bit simpler, but it still involves adding a lot of boilerplate code. I don't really want to pay for things that are irrelevant or unused with code. Ideally, in order to get started with developing a new wayland client type, it should be just enough to initialize the WaylandClient class with a surface. Things such as resizing or moving may not work, but it can be easily fixed by overriding relevant AbstractClient methods. Such an incremental workflow is the best way to achieve support for shell surface protocol %name%, imho.

This merge request provides stub methods for things that are irrelevant for internal, xdg-popup, input-panel, and in the future layer-shell clients.