Stock display (#379)

* Display stock quantity more prominently

* Cleanup search widget

* Update for stock_detail widget

* More tweaks

* Change bottom bar icon

* Display boolean parameters appropriately

* Adds ability to edit part parameters

* Bump icon size a bit

* Improvements to filter options

- Allow filtering by "option" type
- To start with, filter stock by status code

* Remove debug message

* Remove getTriState method

- No longer needed
- Remove associated unit tests

* Adjust filters based on server API version

* Muted colors

lib/preferences.dart

@@ -126,7 +126,12 @@ class InvenTreeSettingsManager {
 
   Future<dynamic> getValue(String key, dynamic backup) async {
 
-    final value = await store.record(key).get(await _db);
+    dynamic value = await store.record(key).get(await _db);
+
+    // Retrieve value
+    if (value == "__null__") {
+      value = null;
+    }
 
     if (value == null) {
       return backup;
@@ -148,32 +153,11 @@ class InvenTreeSettingsManager {
     }
   }
 
-  // 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";
+    value ??= "__null__";
 
     await store.record(key).put(await _db, value);
   }