Sets the snap edge of a handlebox. The snap edge is the edge of the detached child that must be aligned with the corresponding edge of the "ghost" left behind when the child was detached to reattach the torn-off window. Usually, the snap edge should be chosen so that it stays in the same place on the screen when the handlebox is torn off.
If the snap edge is not set, then an appropriate value will be guessed from the handle position. If the handle position is Gtk::POS_RIGHT or Gtk::POS_LEFT, then the snap edge will be Gtk::POS_TOP, otherwise it will be Gtk::POS_LEFT.
To unset the value (and let Gtk guess the position), set it to -1.
See also: get_snap_edge()