Update layers panel on programatic call to createLayer.

Add class=“layer” to each layer element. This happens when calling
createLayer() or any call to identifyLayers(). This happens with new
drawings and when opening legacy drawings, so it’s fully backwards
compatible.
svg-editor.js elementChanged() looks for g.layer as an addition test
for calling populateLayers() which updates the layers panel.
Addition tests for class=“layer” added to draw_test.html.
Fixed Firefox exception in draw_test.html.

editor/svg-editor.js

@@ -1835,6 +1835,15 @@ TODOS
 				});
 			};
 
+			/**
+			 * Test whether an element is a layer or not.
+			 * @param {SVGGElement} elem - The SVGGElement to test.
+			 * @returns {boolean} True if the element is a layer
+			 */
+			function isLayer(elem) {
+				return elem && elem.tagName === 'g' && svgedit.LAYER_CLASS_REGEX.test(elem.getAttribute('class'))
+			}
+
 			// called when any element has changed
 			var elementChanged = function(win, elems) {
 				var i,
@@ -1846,10 +1855,13 @@ TODOS
 				for (i = 0; i < elems.length; ++i) {
 					var elem = elems[i];
 
-					// if the element changed was the svg, then it could be a resolution change
-					if (elem && elem.tagName === 'svg') {
+					var isSvgElem = (elem && elem.tagName === 'svg');
+					if (isSvgElem || isLayer(elem)) {
 						populateLayers();
-						updateCanvas();
+						// if the element changed was the svg, then it could be a resolution change
+						if (isSvgElem) {
+							updateCanvas();
+						}
 					}
 					// Update selectedElement if element is no longer part of the image.
 					// This occurs for the text elements in Firefox