Support multiple text-input protocols at once in Qt Wayland Compositor

Different clients may support different text-input protocols for receiving input from the compositor. Right now, the compositor can only support a single interface and the clients have to also support this. But being able to instantiate multiple interfaces in the same compositor should mainly be a case of plumbing. Doing this would also match our approach to shell extensions and client buffer integrations, where different clients can attach to different extensions.