Move scoped_model_counter to provider_counter (#82)

I used `flutter create` anew, so the project structure now has additional 2 `AndroidManifest.xml` files. Everything else stayed the same, so from git’s perspective, it’s just moved files.

provider_counter/lib/main.dart

@@ -0,0 +1,88 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+void main() {
+  // Initialize the model. Can be done outside a widget, like here.
+  var counter = Counter();
+
+  // Setup a delayed interaction with the model (increment each 5 seconds),
+  // outside of the Flutter widget tree.
+  Timer.periodic(
+    const Duration(seconds: 5),
+    (timer) => counter.increment(),
+  );
+
+  // Now we're ready to run the app...
+  runApp(
+    // ... and provide the model to all widgets within.
+    ChangeNotifierProvider<Counter>.value(
+      notifier: counter,
+      child: MyApp(),
+    ),
+  );
+}
+
+/// Simplest possible model, with just one field.
+class Counter extends ChangeNotifier {
+  int value = 0;
+
+  void increment() {
+    value += 1;
+    notifyListeners();
+  }
+}
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Flutter Demo',
+      theme: ThemeData(
+        primarySwatch: Colors.blue,
+      ),
+      home: MyHomePage(),
+    );
+  }
+}
+
+class MyHomePage extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Flutter Demo Home Page'),
+      ),
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: <Widget>[
+            Text('You have pushed the button this many times:'),
+            // ScopedModelDescendant looks for an ancestor ScopedModel widget
+            // and retrieves its model (Counter, in this case).
+            // Then it uses that model to build widgets, and will trigger
+            // rebuilds if the model is updated.
+            Consumer<Counter>(
+              builder: (context, counter, child) => Text(
+                    '${counter.value}',
+                    style: Theme.of(context).textTheme.display1,
+                  ),
+            ),
+          ],
+        ),
+      ),
+      floatingActionButton: FloatingActionButton(
+        // ScopedModel.of is another way to access the model object held
+        // by an ancestor ScopedModel. By default, it just returns
+        // the current model and doesn't automatically trigger rebuilds.
+        // Since this button always looks the same, though, no rebuilds
+        // are needed.
+        onPressed: () =>
+            Provider.of<Counter>(context, listen: false).increment(),
+        tooltip: 'Increment',
+        child: Icon(Icons.add),
+      ),
+    );
+  }
+}