implement Qt Quick nested windows as decorated Items

It's becoming more common (and even more so on Wayland) for windows to draw their own window decorations rather than generally leaving it up to the window manager. This of course is against good design and classic usability principles though, in same way as using custom components instead of standardized widgets: if every window consists of 100% custom skin, the overall desktop starts to look too cluttered and inconsistent and confusing. So from that perspective, even if windows CAN draw their own decorations, they SHOULD mostly reuse the same code to do so, which is more efficient and leads to a common theme across all the windows on the desktop. But this task is about enabling self-decorating windows rather than setting policy against it.

So for the sake of having a common set of window decorations ahead of time (and perhaps stave off a proliferation of them in every QML application), there has been some interest in providing them in Qt Controls.

So far we are talking about decorating top-level windows, but there are a few embedded platforms that don't support multiple top-level windows. So now we have a justification for doing this in the first place: it would be useful to simulate multiple windows on such platforms. The first use case that has come up is that if a dialog is going to pop up on such a platform, it must be an Item rather than a Window; but so far the user has no way to move or resize the fake window: it just gets anchored to the center of the screen, or perhaps fills the whole screen. If there were a set of reusable QML-based window decorations, they could be applied as a wrapper to this type of fake window.