WelcomePage: Fix scrolling logic

We only need one scrollable element in the WelcomePage. By passing
the height properties of inner elements to the outermost one we can
avoid multiple vertical scrollbars next to each other. Since this
removes the QScrollArea around the welcome page it fixes
QTCREATORBUG-10796

Task-number: QTCREATORBUG-10796

Change-Id: I08b0409e21f16be6c3e0e5a58f2665320d0a14fe
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>