add geometry to QPlatformDialogHelpers

This comes up in the context of the QtQuick.Dialogs module: it's a bit unexpected to create a graphical item in Qt Quick and then have no control over its geometry. A dialog can even on rare occasions be instantiated as an Item instead of a Window (if the platform doesn't support multiple top-level windows), in which case the application writer could really reasonably expect to be able to control its size and position, as with every other Item. But we don't provide this ability in the Dialogs module because the platform dialog helpers don't provide it, and usually that's the kind of dialog which will actually be instantiated.

On the one hand, the windowing system usually positions dialogs reasonably well (especially if they are transient for the main window), so the the app writer really shouldn't care. But on the other hand, the API looks incomplete without the ability to at least query the geometry, and possibly even control it.