wayland: Add present_unconditional() to gtk-shell protocol
Most of the commits are from !275 (closed) and !258 (closed). The difference is that a present_unconditional() request is added (as suggested by @matthiasc) and the deprecation of gtk_window_present() is removed (we can talk about that later, but I don't think it's right to add a request only to be deprecated).
This partially fixes #624 (closed).