mobile: Add about screen

mobile/lib/screens/about.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_markdown/flutter_markdown.dart';
+import 'package:url_launcher/url_launcher.dart' as url;
+
+class AboutScreen extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final textTheme = theme.textTheme;
+
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('About'),
+      ),
+      body: FutureBuilder<String>(
+        future: rootBundle.loadString('assets/about.md'),
+        builder: (context, snapshot) {
+          if (snapshot.hasData) {
+            return Markdown(
+              data: snapshot.data,
+              styleSheet: MarkdownStyleSheet(
+                h1: textTheme.headline6.copyWith(
+                  height: 2.0,
+                ),
+                a: textTheme.bodyText1.copyWith(
+                  color: theme.accentColor,
+                  decoration: TextDecoration.underline,
+                ),
+              ),
+              onTapLink: (link) => url.launch(link),
+            );
+          } else {
+            return Container();
+          }
+        },
+      ),
+    );
+  }
+}

mobile/lib/screens/home.dart

@@ -4,6 +4,7 @@ import 'package:musicus_database/musicus_database.dart';
 
 import '../icons.dart';
 
+import 'about.dart';
 import 'person.dart';
 import 'settings.dart';
 
@@ -48,6 +49,10 @@ class _HomeScreenState extends State<HomeScreen> {
                 value: 2,
                 child: Text('Settings'),
               ),
+              PopupMenuItem(
+                value: 3,
+                child: Text('About'),
+              ),
             ],
             onSelected: (selected) {
               if (selected == 1) {
@@ -65,6 +70,13 @@ class _HomeScreenState extends State<HomeScreen> {
                     builder: (context) => SettingsScreen(),
                   ),
                 );
+              } else if (selected == 3) {
+                Navigator.push(
+                  context,
+                  MaterialPageRoute(
+                    builder: (context) => AboutScreen(),
+                  ),
+                );
               }
             },
           ),