Revert "gtk2: set_cursor is a property."

This reverts commit bc7244240b1ddc7e10218642be5465f78414d14d.
gtk2: In Gdk::Window, set_cursor is a property since gtk+2 2.18.0.

see: https://developer.gnome.org/gdk2/stable/gdk2-Windows.html#GdkWindow--cursor