Welcome page: Set size constraint on the container widget.

Such that QLayout properly handles it. Fixes warnings about
resizing violating size constraints on Windows.

Task-number: QTCREATORBUG-10251

Change-Id: I25361fbc994f23d5616c240663d64f8b3474e7f7
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>