GTK+ 3.22 to Fully Support Wacom Tablets on Wayland, Switch to XDG-Shell 6

We will start with the Wayland improvements, because this is the next generation display server that will soon be used by default on almost all GNU/Linux distributions, starting with the upcoming Fedora 25 Linux operating system, which implemented Wayland by default for the GNOME Live CD (a.k.a. Fedora Workstation). Therefore, GTK+ 3.21.5 comes with full support for Wacom tablets on Wayland by making use of the tablet pad protocol, will switch to the sixth generation of the xdg-shell protocol, improve menu positioning and handling of the state of a tiled window, as well as to export window handles via the xdg_foreign protocol.