0.17: Add "initial_commit" property to XdgSurface

[heuer]

XdgSurface does not expose the "initial_commit" boolean property, see https://wayland.emersion.fr/wlroots/wlr/types/wlr_xdg_shell.h.html#struct-wlr_xdg_surface

Current workaround: xdg_surface._ptr.initial_commit