Add keys to change image source in sequence mode

Add the keys 'j' and 'k' to change the image source in the viewer.
- 'j': previous source
- 'k': next source

src/viewer.js

@@ -2782,6 +2782,12 @@ function onCanvasKeyPress( event ) {
               this.viewport.toggleFlip();
               event.preventDefault = true;
               break;
+            case 106: //j - previous image source
+              onPrevious.bind(this)();
+              break;
+            case 107: //k - next image source
+              onNext.bind(this)();
+              break;
             default:
                 // console.log( 'navigator keycode %s', event.keyCode );
                 event.preventDefault = false;