Defer QPlatformWindow creation on setVisible(true) if parent hasn't been created

When a child QWindow is shown by calling setVisible(true), we don't need to
create the platform window immediately if the parent window hasn't been
created yet.

We defer creation until the parent is created, or we're re-parented into
a created parent or made top level.

This optimization is more important now that we create the full parent
hierarchy once we decide that we need to create a child QWindow.

Change-Id: Ia4f0430f0d3709a12f41f6473c1cea6b0ef3c9cd
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>