PO Contact (#305)

* Bug fix in API forms

- Allow form fields to specify custom filters at runtime

* Add "contact" model to purchase order edit form

* Add action to create new purchase order from list widget

* widget updates for purchase order

lib/inventree/purchase_order.dart

@@ -26,11 +26,21 @@ class InvenTreePurchaseOrder extends InvenTreeModel {
   Map<String, dynamic> formFields() {
     return {
       "reference": {},
+      "supplier": {
+        "filters": {
+          "is_supplier": true,
+        },
+      },
       "supplier_reference": {},
       "description": {},
       "target_date": {},
       "link": {},
       "responsible": {},
+      "contact": {
+        "filters": {
+          "company": supplierId,
+        }
+      },
     };
   }