QmlDesigner.PropertyEditor: Fixing minimum and maximum values for reals

I also show 2 decimals for the template based real SpinBox.

Task-number: QTCREATORBUG-12041
Change-Id: Iac79dec47c9ec55913231e327ba2727e0af5c652
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Robert Loehning <robert.loehning@digia.com>