WebAssembly: QWindow as html element

Today, QScreen is a first-class html element: either a canvas or a container element. The application's windows are not present in the html structure. This is unlike other platforms where QWindow is backed by some native window/view type.

We can implement a mode where each QWindow is backed by a native html element:

• Qt windows: a canvas element, which the browser will composit. This enables mixing raster and OpenGL QWindows.
• Foreign windows: a video or iframe element, for QtMultimedia and QWebView use cases

What about QScreen? Two options:

• The container elements(s) provided to qtloader becomes the QScreen(s)
• Implicit QScreen: the top-level body element is the (one and only) QScreen