0.17: Provide wlr_surface.mapped property

compositor.Surface does not expose the boolean ```mapped``` property, see https://wayland.emersion.fr/wlroots/wlr/types/wlr_compositor.h.html#struct-wlr_surface

Workaround: ```surface._ptr.mapped```

Maybe pywlroots should expose the property as ``ìs_mapped``` to indicate a boolean?