Major overhaul of "paginated list" widget class

- Simplify implementation
- Create mixin class for code reuse
- Allow custom app-bar
- Allow custom ordering / sorting options
- Improve code commenting / readability

lib/widget/part_detail.dart

@@ -354,7 +354,9 @@ class _PartDisplayState extends RefreshableState<PartDetailWidget> {
                   Navigator.push(
                     context,
                     MaterialPageRoute(
-                      builder: (context) => BomList(part)
+                      builder: (context) => PaginatedBomList({
+                        "part": part.pk.toString(),
+                      })
                     )
                   );
                 }