Move MarkdownToolbar to its own file

lib/widgets/markdown_toolbar.dart

@@ -0,0 +1,33 @@
+import 'package:flutter/material.dart';
+
+class MarkdownToolBar extends StatelessWidget {
+  final Function onHeader1;
+  final Function onItallics;
+  final Function onBold;
+
+  MarkdownToolBar({
+    @required this.onHeader1,
+    @required this.onItallics,
+    @required this.onBold,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      children: [
+        IconButton(
+          icon: const Text('H1'),
+          onPressed: onHeader1,
+        ),
+        IconButton(
+          icon: const Text('I'),
+          onPressed: onItallics,
+        ),
+        IconButton(
+          icon: const Text('B'),
+          onPressed: onBold,
+        ),
+      ],
+    );
+  }
+}