Allow to remove callbacks by setting their value to null
Allow to remove callbacks by setting their value to null

vsajip/svg-pan-zoom,Paulyoufu/svg-pan-zoom,praneybehl/svg-pan-zoom,ariutta/svg-pan-zoom,etyplt/svg-pan-zoom,Paulyoufu/svg-pan-zoom,vsajip/svg-pan-zoom,etyplt/svg-pan-zoom,lsloss/svg-pan-zoom,lsloss/svg-pan-zoom,ariutta/svg-pan-zoom,praneybehl/svg-pan-zoom,ariutta/svg-pan-zoom