Added Buyable interface to IBuyableBehaviour and added image for Dexterity content types

- Added IBuyable to class IByableBehaviour so the viewlet shows when you add Dexterity Behaviour
- added image for dexterity content, 
- and also for ContentLeadimage although I dont see why we need that (does dexterity content types have contentleadimage and if yes, why not just add an image field?)