QWasmWindow implementation cleanup

QWasmWindow today has for the most part the behavior we want, but has gone through several transitons (e.g. canvas per screen -> canvas per window) which has left the code with some need for cleanup and refactoring.

Tasks include:

• Facilitate adding support for QWindow::fromWinId() (which native element is "the window"?)
• Minimize usage of html elements
• Add "window manager" elements lazily (i.e. not for "fullscreen" windows)
• Move "contentEditable" away from the canvas
• Move all client event handlers to the window element.

Current screen/window html element tree:

Target screen/window html element tree: