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* Remove outdated filtering preferences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/purchase_order_list.dart

@@ -1,4 +1,5 @@
 import "package:flutter/material.dart";
+import "package:font_awesome_flutter/font_awesome_flutter.dart";
 
 import "package:inventree/inventree/company.dart";
 import "package:inventree/inventree/model.dart";
@@ -29,32 +30,43 @@ class _PurchaseOrderListWidgetState extends RefreshableState<PurchaseOrderListWi
 
   final Map<String, String> filters;
 
+  bool showFilterOptions = false;
+
   @override
   String getAppBarTitle(BuildContext context) => L10().purchaseOrders;
 
+  @override
+  List<Widget> getAppBarActions(BuildContext context) => [
+    IconButton(
+      icon: FaIcon(FontAwesomeIcons.filter),
+      onPressed: () async {
+        setState(() {
+          showFilterOptions = !showFilterOptions;
+        });
+      },
+    )
+  ];
+
   @override
   Widget getBody(BuildContext context) {
-    return PaginatedPurchaseOrderList(filters);
+    return PaginatedPurchaseOrderList(filters, showFilterOptions);
   }
 }
 
 
-class PaginatedPurchaseOrderList extends StatefulWidget {
+class PaginatedPurchaseOrderList extends PaginatedSearchWidget {
 
-  const PaginatedPurchaseOrderList(this.filters);
-
-  final Map<String, String> filters;
+  const PaginatedPurchaseOrderList(Map<String, String> filters, bool showSearch) : super(filters: filters, showSearch: showSearch);
 
   @override
-  _PaginatedPurchaseOrderListState createState() => _PaginatedPurchaseOrderListState(filters);
+  _PaginatedPurchaseOrderListState createState() => _PaginatedPurchaseOrderListState();
 
 }
 
 
 class _PaginatedPurchaseOrderListState extends PaginatedSearchState<PaginatedPurchaseOrderList> {
 
-  _PaginatedPurchaseOrderListState(Map<String, String> filters) : super(filters);
-
+  _PaginatedPurchaseOrderListState() : super();
   // Purchase order prefix
   String _poPrefix = "";