Add hooks to the merge PO feature.

In the core OpenERP purchase module, there is a wizard to merge purchase orders. That feature is convenient, but as soon as a field is added to the purchase order, it does not work anymore and needs to be patched. The original implementation does not provide any hooks for extension, and modules can only reimplement a method completely. This required a lot of copy and paste, and worse, it breaks if two modules attempt to do that.

Therefore, this module reimplements the feature, with the same basic result in the standard case. Hooks are provided for extra modules that add fields or change the logic.