Set menu window type hint to DIALOG

Some tiling window managers, like e.g. i3wm, rely on window type hint to determine whether to tile or float a particular window. Windows with hint NORMAL (the default) are thus usually tiled, and dialogs or small or temporary windows, menus, and notifications are more convenient floating.

Reference: http://standards.freedesktop.org/wm-spec/latest/ar01s05.html#idp1333344