Add LinearProgressIndicator for background queries

flutter/lib/components/layout/app_bar_progress_indicator.dart

@@ -0,0 +1,21 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+import 'package:simplecloudnotifier/state/app_bar_state.dart';
+
+class AppBarProgressIndicator extends StatelessWidget implements PreferredSizeWidget {
+  @override
+  Size get preferredSize => Size(double.infinity, 1.0);
+
+  @override
+  Widget build(BuildContext context) {
+    return Consumer<AppBarState>(
+      builder: (context, value, child) {
+        if (value.loadingIndeterminate) {
+          return LinearProgressIndicator(value: null);
+        } else {
+          return SizedBox.square(dimension: 4); // 4 height is the same as the LinearProgressIndicator
+        }
+      },
+    );
+  }
+}