Implement click through windows

[efyang]

http://stackoverflow.com/questions/11914785/how-to-click-through-gtk-window

[efyang]

http://gtk-rs.org/docs/gdk_sys/fn.gdk_window_input_shape_combine_region.html
http://gtk-rs.org/docs/cairo_sys/fn.cairo_region_create.html
https://people.gnome.org/~gcampagna/docs/Gdk-3.0/Gdk.Window.input_shape_combine_region.html