wayland: do not require wl_shell
Most of the time when cogl is used, the app doesn't really need this functionality (i.e it manages the app window on its own). So requiring wl_shell turns out to prevent apps using for ex. xdg-shell from working on compositors that do not support wl_shell.
This should also be backported to the cogl-1.22
branch, let me know if I should open another MR for that.