xdg: destroy foreign toplevel handle on unmap

xdg: destroy foreign toplevel handle on unmap

xdg-shell protocol says:
    All active operations (e.g., move, resize) are canceled and all
    attributes (e.g. title, state, stacking, ...) are discarded for an
    xdg_toplevel surface when it is unmapped.
So, when a xdg-toplevel is unmapped (not minimized), the corresponding
foreign handler should be destroyed to reset attributes.