Add picker for post sorting

lib/widgets/sort_posts_picker.dart

@@ -0,0 +1,71 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_hooks/flutter_hooks.dart';
+import 'package:lemmur/widgets/bottom_modal.dart';
+import 'package:lemmy_api_client/lemmy_api_client.dart';
+
+class SortPostsPicker extends HookWidget {
+  final Function(SortType sort) onChange;
+  final SortType defaultSort;
+
+  SortPostsPicker({
+    @required this.onChange,
+    this.defaultSort = SortType.active,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    var sort = useState(defaultSort);
+
+    void selectSortType(BuildContext context) {
+      showModalBottomSheet(
+        isScrollControlled: true,
+        context: context,
+        backgroundColor: Colors.transparent,
+        builder: (context) => BottomModal(
+            title: 'sort by',
+            child: Column(
+              children: [
+                for (var x in SortType.values)
+                  RadioListTile<SortType>(
+                    value: x,
+                    groupValue: sort.value,
+                    title: Text(x.toString()),
+                    onChanged: (val) {
+                      sort.value = val;
+                      onChange(val);
+                      Navigator.of(context).pop();
+                    },
+                  ),
+              ],
+            )),
+      );
+    }
+
+    return Padding(
+      padding: const EdgeInsets.symmetric(horizontal: 15, vertical: 10),
+      child: Row(
+        children: [
+          OutlineButton(
+            shape: RoundedRectangleBorder(
+              borderRadius: BorderRadius.circular(10),
+            ),
+            onPressed: () => selectSortType(context),
+            child: Row(
+              mainAxisSize: MainAxisSize.min,
+              children: <Widget>[
+                Text(sort.value.toString()),
+                const SizedBox(width: 8),
+                Icon(Icons.arrow_drop_down),
+              ],
+            ),
+          ),
+          Spacer(),
+          IconButton(
+            icon: Icon(Icons.view_stream),
+            onPressed: () => print('TBD'),
+          )
+        ],
+      ),
+    );
+  }
+}