mobile: Fine tune account settings

If the user enters something into the login fields, the values will be
copied to the registration screen. Also, the password is required to
delete an account now.

mobile/lib/screens/delete_account.dart

@@ -0,0 +1,96 @@
+import 'package:flutter/material.dart';
+import 'package:musicus_common/musicus_common.dart';
+
+class DeleteAccountScreen extends StatefulWidget {
+  @override
+  _DeleteAccountScreenState createState() => _DeleteAccountScreenState();
+}
+
+class _DeleteAccountScreenState extends State<DeleteAccountScreen> {
+  final _passwordController = TextEditingController();
+
+  bool _loading = false;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Delete account'),
+        actions: <Widget>[
+          Builder(
+            builder: (context) {
+              if (_loading) {
+                return Padding(
+                  padding: const EdgeInsets.all(16.0),
+                  child: Center(
+                    child: SizedBox(
+                      width: 24.0,
+                      height: 24.0,
+                      child: CircularProgressIndicator(
+                        strokeWidth: 2.0,
+                      ),
+                    ),
+                  ),
+                );
+              } else {
+                return FlatButton(
+                  onPressed: () async {
+                    final backend = MusicusBackend.of(context);
+
+                    if (_passwordController.text ==
+                        backend.settings.account.value.password) {
+                      setState(() {
+                        _loading = true;
+                      });
+
+                      await backend.client.deleteAccount();
+                      await backend.settings.clearAccount();
+
+                      setState(() {
+                        _loading = false;
+                      });
+
+                      Navigator.pop(context);
+                    } else {
+                      Scaffold.of(context).showSnackBar(SnackBar(
+                        content: Text('Wrong password'),
+                      ));
+                    }
+                  },
+                  child: Text('DELETE'),
+                );
+              }
+            },
+          ),
+        ],
+      ),
+      body: ListView(
+        children: <Widget>[
+          Padding(
+            padding: const EdgeInsets.only(
+              left: 16.0,
+              right: 16.0,
+              top: 16.0,
+              bottom: 8.0,
+            ),
+            child: Text(
+              'If you really want to delete your account, enter your password '
+              'below.',
+              style: Theme.of(context).textTheme.subtitle1,
+            ),
+          ),
+          Padding(
+            padding: const EdgeInsets.all(16.0),
+            child: TextField(
+              controller: _passwordController,
+              obscureText: true,
+              decoration: InputDecoration(
+                labelText: 'Password',
+              ),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}