'DOMNodeInserted' is deprecated and does not work in Gallery Modal.

[jsomeara]

Brief description of problem/feature

I noticed this issue when I was adding keyboard shortcuts to the gallery expanded view.

Opening the gallery in Chrome, I get the following error:

jquery-1.12.2.min.js:3 [Deprecation] Listener added for a 'DOMNodeInserted' mutation event. Support for this event type has been removed, and this event will no longer be fired. See https://chromestatus.com/feature/5083947249172480 for more information.

This occurs because of the following lines in Modal.js:

        // Google Street View loads inside 'actual-pano' but there is no event triggered after it loads all the components.
        // So we need to detect it by brute-force.
        $('.actual-pano').bind('DOMNodeInserted', function(e) {
            if (e.target && e.target.className && typeof e.target.className === 'string' && e.target.className.indexOf('widget-scene-canvas') > -1) {
                $('.widget-scene-canvas').bind('mousedown', handlerViewControlLayerMouseDown).bind('mouseup', handlerViewControlLayerMouseUp);
            }
        });

Expected behavior

No error should occur, and it should properly detect the DOM changes.

Current behavior (add screenshots if applicable; check terminal & browser console for errors)

Steps to reproduce

Open the gallery page while the developer tools console is open.

Potential solution(s)

Use mutation observer instead of the deprecated DOMNodeInserted event.

[jsomeara]

Made a PR: #3617