Fix QPlatformWindow::initialGeometry() to not touch large windows.

Do not touch windows whose geometry (including the  unknown frame
size) is likely to be larger than the screen.

Remove fix-up in the Windows plugin.

Task-number: QTBUG-30142
Task-number: QTBUG-31071
Change-Id: I13a8ffb9fb9d8c71d35de75094275388fa427f2c
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>