Support having a QtObject as the root element

You may have a system ui that renders to multiple windows.
In that scenario you would like your root qml file to look like this:

QtObject {
    Window {}
    Window {}
    [...]
}

But right now you have to resort to the following structure, which looks rather hacky (like what's being done in Neptune 2 and 3):

Window {
    Window {}
    [...]
}