Setting parents for WASM platform windows is now supported. This means
that windows now reside in a hierarchical window tree, with the screen
and individual windows being nodes (QWasmWindowTreeNode), each
maintaining their own child window stack.
The divs backing windows are properly reparented in response to Qt
window parent changes, so that the html structure reflects what is
happening in Qt.
Change-Id: I55c91d90caf58714342dcd747043967ebfdf96bb
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
