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* Remove outdated filtering preferences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/preferences.dart

@@ -16,9 +16,6 @@ const String INV_HOME_SHOW_SUPPLIERS = "homeShowSuppliers";
 const String INV_SOUNDS_BARCODE = "barcodeSounds";
 const String INV_SOUNDS_SERVER = "serverSounds";
 
-const String INV_PART_SUBCATEGORY = "partSubcategory";
-
-const String INV_STOCK_SUBLOCATION = "stockSublocation";
 const String INV_STOCK_SHOW_HISTORY = "stockShowHistory";
 
 const String INV_REPORT_ERRORS = "reportErrors";
@@ -86,6 +83,11 @@ class InvenTreeSettingsManager {
 
   Future<Database> get _db async => InvenTreePreferencesDB.instance.database;
 
+
+  Future<void> removeValue(String key) async {
+    await store.record(key).delete(await _db);
+  }
+
   Future<dynamic> getValue(String key, dynamic backup) async {
 
     final value = await store.record(key).get(await _db);
@@ -103,13 +105,40 @@ class InvenTreeSettingsManager {
 
     if (value is bool) {
       return value;
+    } else if (value is String) {
+      return value.toLowerCase().contains("t");
     } else {
-      return backup;
+      return false;
     }
   }
 
+  // Load a tristate (true / false / null) setting
+  Future<bool?> getTriState(String key, dynamic backup) async {
+    final dynamic value = await getValue(key, backup);
+
+    if (value == null) {
+      return null;
+    } else if (value is bool) {
+      return value;
+    } else {
+      String s = value.toString().toLowerCase();
+
+      if (s.contains("t")) {
+        return true;
+      } else if (s.contains("f")) {
+        return false;
+      } else {
+        return null;
+      }
+    }
+  }
+
+  // Store a key:value pair in the database
   Future<void> setValue(String key, dynamic value) async {
 
+    // Encode null values as strings
+    value ??= "null";
+
     await store.record(key).put(await _db, value);
   }