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* Remove outdated filtering preferences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

lib/widget/refreshable_state.dart

@@ -9,9 +9,7 @@ import "package:flutter/material.dart";
 mixin BaseWidgetProperties {
 
   // Return a list of appBar actions (default = None)
-  List<Widget> getAppBarActions(BuildContext context) {
-    return [];
-  }
+  List<Widget> getAppBarActions(BuildContext context) => [];
 
   // Return a title for the appBar
   String getAppBarTitle(BuildContext context) { return "--- app bar ---"; }