Revert "Revert "add back wayland 1.5 for cogl 1.18""

This reverts commit 2e380c0070c3e22d2f17ed91e82e29a4d9d9cfca.

Plasma 5.1 *requires* Wayland >=1.3 and <1.6.
plasma-5.1: use wayland 1.5