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* Remove outdated filtering preferences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/api_form.dart

@@ -704,7 +704,8 @@ class APIFormField {
       labelStyle: _labelStyle(),
       helperText: helpText,
       helperStyle: _helperStyle(),
-      initial: value as bool,
+      initial: value as bool?,
+      tristate: (getParameter("tristate") ?? false) as bool,
       onSaved: (val) {
         data["value"] = val;
       },