ProjectMode: Set only maximum width instead of fixed width

Seems to be a improvement in resizing behavior.

Task-number: QTCREATORBUG-11286
Change-Id: Iac19a76265c98861e4da9d23c3e5fb3a36f0386e
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>