Fix bool fields (#778)

* Improved UX for boolean fields

- Use segmented button
- Allow tristate
- Improved filtering options

* Bug fix for null filter values

* Prevent null filters from being sent to the server

* Update release notes

lib/labels.dart

@@ -45,6 +45,8 @@ class LabelFormWidgetState extends APIFormWidgetState {
     if (field == "plugin") {
       onPluginChanged(value.toString());
     }
+
+    super.onValueChanged(field, value);
   }
 
   @override