List refactor (#179)

* Catch paginator bug if widget is disposed before request returns

* Refactoring paginated query widget

- Add option to enable / disable search filters

* Major refactor of paginated search widget

- Learned something new.. a state can access widget.<attribute>
- THIS CHANGES EVERTHING

* Preferences: Add code for tri-state values

- Also improve unit testing for preferences code

* Allow boolean form fields to be optionally tristate

* paginator: Allow custom boolean filters

* Remove outdated filtering preferences

* Refactor filter options

- Allow specification of more detailed options

* Add custom filters for "part" list

* filter tweaks

* Remove legacy "SublocationList" widget

* Add filtering option for locationlist

* Updates for stock location widget

* Refactor category display widget

* More widget refactoring

* Update main search widget

* Fix unit tests

* Improve filtering on BOM display page

test/models_test.dart

@@ -63,7 +63,11 @@ void main() {
 
       // List *all* parts
       results = await InvenTreePart().list();
-      assert(results.length == 13);
+      expect(results.length, equals(14));
+
+      // List with active filter
+      results = await InvenTreePart().list(filters: {"active": "true"});
+      expect(results.length, equals(13));
 
       for (var result in results) {
         // results must be InvenTreePart instances