Allow QQuickPopup to create a top-level window, instead of the QQuickPopupItem

The popup in QQC2, as well as other popup types (Tooltip, Drawer, Menu, Dialog), create an item in the scene (QQuickPopupItem), instead of a top level window, to display the content inside of.

This can feel off on desktop systems, especially when showing a dialog to a user.

In Qt Widgets, dialogs are always top-level windows, and feel much better to use on desktop systems, than the QQC2 Dialog.

QQC2 Popup needs to have the option to choose between creating a QQuickPopupItem or a Window, to add its content to.

Adding this logic to QQuickPopup would allow tooltips and menus to appear outside the application's window, which could be useful on some circumstances. But dialogs are clearly the largest elephant in the room.