libweston-desktop: Bump xdg-shell to version 3

xdg_popup::reposition is the main addition in this version, it allows popups to be repositioned based on a new xdg_positioner object at the client’s will.

Reactive, parent_size and parent_configure are set but ignored, and tiling enums from version 2 are never emitted.

This fixes blinking menus in gtk4-demo, and bumps their maximum 60 fps number (on my computer) from ~10 to ~60: fishbowl-menubutton

Signed-off-by: Emmanuel Gil Peyrot linkmauve@linkmauve.fr