Added const enabledPlug() method to dependency node binding
Added const enabledPlug() method to dependency node binding

The method's absence was making python overrides of enabledPlug() inaccessible in certain situations

appleseedhq/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,boberfly/gaffer,chippey/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,paulondc/gaffer,johnhaddon/gaffer,hradec/gaffer,GafferHQ/gaffer,hradec/gaffer,boberfly/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,davidsminor/gaffer,johnhaddon/gaffer,paulondc/gaffer,cedriclaunay/gaffer,hradec/gaffer,goddardl/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,cedriclaunay/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,cedriclaunay/gaffer,davidsminor/gaffer,goddardl/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,hradec/gaffer,lucienfostier/gaffer,appleseedhq/gaffer,hradec/gaffer,appleseedhq/gaffer,goddardl/gaffer,chippey/gaffer,ivanimanishi/gaffer,boberfly/gaffer,davidsminor/gaffer,paulondc/gaffer,andrewkaufman/gaffer,chippey/gaffer,goddardl/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,davidsminor/gaffer,paulondc/gaffer,cedriclaunay/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,chippey/gaffer,boberfly/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer