NoteBrowser: Allow browsing via swipes

This is the first prototype for swiping. It really needs a lot of work,
and it needs some kind of animation.

lib/note_viewer.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
+import 'package:journal/widgets/swipe_detector.dart';
 
 import 'note.dart';
 
@@ -64,7 +65,7 @@ class NoteViewer extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return new SingleChildScrollView(
+    var view = new SingleChildScrollView(
       child: new Column(
         children: <Widget>[
           _buildHeader(context),
@@ -75,6 +76,12 @@ class NoteViewer extends StatelessWidget {
       ),
       padding: const EdgeInsets.all(16.0),
     );
+
+    return new SwipeDetector(
+      child: view,
+      onLeftSwipe: showNextNoteFunc,
+      onRightSwipe: showPrevNoteFunc,
+    );
   }
 
   Widget _buildHeader(BuildContext context) {

lib/widgets/swipe_detector.dart

@@ -0,0 +1,33 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/foundation.dart';
+
+class SwipeDetector extends StatelessWidget {
+  final VoidCallback onLeftSwipe;
+  final VoidCallback onRightSwipe;
+  final Widget child;
+
+  SwipeDetector({
+    @required this.onLeftSwipe,
+    @required this.onRightSwipe,
+    @required this.child,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    double primaryDelta;
+
+    return new GestureDetector(
+      child: child,
+      onHorizontalDragUpdate: (DragUpdateDetails details) {
+        primaryDelta = details.primaryDelta;
+      },
+      onHorizontalDragEnd: (DragEndDetails _) {
+        if (primaryDelta > 0) {
+          onRightSwipe();
+        } else {
+          onLeftSwipe();
+        }
+      },
+    );
+  }
+}