QDS-9900 Update the design viewer app to work with Qt 6.5 / emsdk 3.1.25

Use new API in Qt, adapt to changes in Qt and Emscripten.

Incremental step towards Qt 6.6 support (see QDS-9900).

design-viewer/www/index.html

@@ -118,11 +118,7 @@
       </div>
     </div>
 
-    <canvas
-      id="qtcanvas"
-      oncontextmenu="event.preventDefault()"
-      contenteditable="true"
-    ></canvas>
+    <div id="qtcontainer"></div>
     <div id="footer">
       <div id="alertBox" class="alert">
         <span