Make project dir default

The project dir defaults to the home directory.

Task-number: QTCREATORBUG-13366
Change-Id: I5a34ae458cacde9e12df50e0796ca7d236bb123e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Welcome Screen: Fix scrollbar

The size of the project list should only depend on the size of the items,
not on the size of the parent.

Task-number: QTCREATORBUG-13387
Change-Id: Iad8a32999a277532c2b64a7a889ddaafa40f693b
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>