[Xwayland-22.1] Backports a few more fixes

Merged Olivier Fourdan requested to merge ofourdan/xserver:xwayland-backports into xwayland-22.1

This backports:

  • cb33e0d2 - Forbid server grabs by non-WM on rootless XWayland
  • 395c25f1 - build: Bump wayland requirement to 1.18
  • 8a5f3ddb - xwayland: set tag on our surfaces
  • a1d14aa8 - xwayland: Clear the "xwl-window" tag on unrealize

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