PO Line Item Improvements (#340)

* Refactor thumbnail image

* Add paginated list of purchase order line items

* Refactor getBody() function

- No longer "have" to specify
- Can use getTiles for a simpler interface

* Add detail widget for polineitem

* add pricing info

* Receive line items via action button

* tweak color

* update release notes

* linting fixes

lib/widget/part_list.dart

@@ -133,11 +133,7 @@ class _PaginatedPartListState extends PaginatedSearchState<PaginatedPartList> {
       title: Text(part.fullname),
       subtitle: Text(part.description),
       trailing: Text(part.stockString()),
-      leading: InvenTreeAPI().getImage(
-        part.thumbnail,
-        width: 40,
-        height: 40,
-      ),
+      leading: InvenTreeAPI().getThumbnail(part.thumbnail),
       onTap: () {
         Navigator.push(context, MaterialPageRoute(builder: (context) => PartDetailWidget(part)));
       },