eglut_wayland: switch from wl_shell to xdg_shell
The wl_shell protocol is deprecated in favor of xdg_shell and some compositors (e.g. wlroots-based compositors like Sway) have already dropped support for wl_shell.
This pull request replaces wl_shell with xdg_shell. To accomplish this, it requires
wayland-scanner to be available, along with Extra CMake Modules and CMake 2.8.12 for building wiht CMake. It runs
wayland-scanner to generate the necessary source files from protocol XML during build.
It also adds the unstable xdg-decorations protocol to request server-side window decorations, where supported. If this is at all controversial, I can move the commit into a separate PR.
Thanks for considering,