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* Remove outdated filtering preferences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/fields.dart

@@ -142,7 +142,10 @@ class FilePickerDialog {
 
 class CheckBoxField extends FormField<bool> {
   CheckBoxField({
-      String? label, bool initial = false, Function(bool?)? onSaved,
+      String? label,
+      bool? initial = false,
+      bool tristate = false,
+      Function(bool?)? onSaved,
       TextStyle? labelStyle,
       String? helperText,
       TextStyle? helperStyle,
@@ -155,6 +158,7 @@ class CheckBoxField extends FormField<bool> {
             //dense: state.hasError,
             title: label != null ? Text(label, style: labelStyle) : null,
             value: state.value,
+            tristate: tristate,
             onChanged: state.didChange,
             subtitle: helperText != null ? Text(helperText, style: helperStyle) : null,
             contentPadding: EdgeInsets.zero,