get buttons () {
        $.console.warn('Viewer.buttons is deprecated; Please use Viewer.buttonGroup');
        return this.buttonGroup;
    }
Executes the provided callback when the TiledImage is fully loaded. If already loaded,
schedules the callback asynchronously. Otherwise, attaches a one-time event listener
for the 'fully-loaded-change' event.
@param {Function} callback - Function to execute when loading completes
@memberof OpenSeadragon.Viewer.prototype