changelog.txt update for button event signatures
changelog.txt update for button event signatures

LarissaSmith/openseadragon,seanich/openseadragon,daverome/openseadragon,rdlester/openseadragon,janelia-flyem/openseadragon,openseadragon/openseadragon,jmnetus/openseadragon,shaunstanislaus/openseadragon,goofwear/openseadragon,mcanthony/openseadragon,daverome/openseadragon,msalsbery/openseadragon,openseadragon/openseadragon,lidasong2014/openseadragon,Iced-Tea/openseadragon,Iced-Tea/openseadragon,shaunstanislaus/openseadragon,msalsbery/openseadragon,ricochet2200/openseadragon,sul-dlss/openseadragon,seanich/openseadragon,lidasong2014/openseadragon,ricochet2200/openseadragon,TMUND/openseadragon,LarissaSmith/openseadragon,avandecreme/openseadragon,jmnetus/openseadragon,janelia-flyem/openseadragon,goofwear/openseadragon,mcanthony/openseadragon,LafayetteCollegeLibraries/openseadragon,rdlester/openseadragon,TMUND/openseadragon,avandecreme/openseadragon