Implementing controlling of the sorting order

The dialog looks quite ugly right now, but at least everything works. I
can work on making it prettier after this.

lib/widgets/sorting_mode_selector.dart

@@ -0,0 +1,87 @@
+import 'package:flutter/material.dart';
+
+import 'package:easy_localization/easy_localization.dart';
+
+import 'package:gitjournal/core/sorting_mode.dart';
+import 'package:gitjournal/screens/settings_screen.dart';
+
+class SortingModeSelector extends StatefulWidget {
+  final SortingMode selectedMode;
+
+  SortingModeSelector(this.selectedMode);
+
+  @override
+  _SortingModeSelectorState createState() => _SortingModeSelectorState();
+}
+
+class _SortingModeSelectorState extends State<SortingModeSelector> {
+  SortingField field;
+  SortingOrder order;
+
+  @override
+  void initState() {
+    super.initState();
+    field = widget.selectedMode.field;
+    order = widget.selectedMode.order;
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    var children = <Widget>[
+      SettingsHeader(tr('settings.sortingMode.field')),
+      for (var sf in SortingField.options) _buildSortingTile(sf),
+      SettingsHeader(tr('settings.sortingMode.order')),
+      for (var so in SortingOrder.options) _buildSortingOrderTile(so),
+    ];
+
+    return AlertDialog(
+      title: Text(tr("widgets.SortingOrderSelector.title")),
+      content: Column(
+        children: children,
+        mainAxisSize: MainAxisSize.min,
+        mainAxisAlignment: MainAxisAlignment.start,
+        crossAxisAlignment: CrossAxisAlignment.start,
+      ),
+      actions: [
+        OutlineButton(
+          child: Text(tr('settings.cancel')),
+          onPressed: () {
+            Navigator.of(context).pop();
+          },
+        ),
+        OutlineButton(
+          child: Text(tr('settings.ok')),
+          onPressed: () {
+            Navigator.of(context).pop(SortingMode(field, order));
+          },
+        ),
+      ],
+    );
+  }
+
+  RadioListTile<SortingField> _buildSortingTile(SortingField sf) {
+    return RadioListTile<SortingField>(
+      title: Text(sf.toPublicString()),
+      value: sf,
+      groupValue: field,
+      onChanged: (SortingField sf) {
+        setState(() {
+          field = sf;
+        });
+      },
+    );
+  }
+
+  RadioListTile<SortingOrder> _buildSortingOrderTile(SortingOrder so) {
+    return RadioListTile<SortingOrder>(
+      title: Text(so.toPublicString()),
+      value: so,
+      groupValue: order,
+      onChanged: (SortingOrder so) {
+        setState(() {
+          order = so;
+        });
+      },
+    );
+  }
+}