[two_dimensional_scrollables] Refactor Radio widgets in example

Refactors the `two_dimensional_scrollables` example to use a new `RadioGroup` widget. This removes the deprecated `groupValue` and `onChanged` properties from each individual `Radio` button and moves them to the parent `RadioGroup`.

Author: alexskobozev

packages/two_dimensional_scrollables/example/lib/table_view/table_explorer.dart

@@ -56,38 +56,25 @@ class _TableExplorerState extends State<TableExplorer> {
       child: Row(
         children: <Widget>[
           const Spacer(),
-          Radio<TableType>(
-            value: TableType.simple,
+          RadioGroup<TableType>(
             groupValue: _currentExample,
             onChanged: (TableType? value) {
-              setState(() {
-                _currentExample = value!;
-              });
+              if (value == null) return;
+              setState(() => _currentExample = value);
             },
+            child: Row(
+              children: <Widget>[
+                Radio<TableType>(value: TableType.simple),
+                const Text('Simple'),
+                _spacer,
+                Radio<TableType>(value: TableType.merged),
+                const Text('Merged'),
+                _spacer,
+                Radio<TableType>(value: TableType.infinite),
+                const Text('Infinite'),
+              ],
+            ),
           ),
-          const Text('Simple'),
-          _spacer,
-          Radio<TableType>(
-            value: TableType.merged,
-            groupValue: _currentExample,
-            onChanged: (TableType? value) {
-              setState(() {
-                _currentExample = value!;
-              });
-            },
-          ),
-          const Text('Merged'),
-          _spacer,
-          Radio<TableType>(
-            value: TableType.infinite,
-            groupValue: _currentExample,
-            onChanged: (TableType? value) {
-              setState(() {
-                _currentExample = value!;
-              });
-            },
-          ),
-          const Text('Infinite'),
           const Spacer(),
         ],
       ),

packages/two_dimensional_scrollables/example/lib/tree_view/tree_explorer.dart

@@ -50,27 +50,22 @@ class _TreeExplorerState extends State<TreeExplorer> {
       child: Row(
         children: <Widget>[
           const Spacer(),
-          Radio<TreeType>(
-            value: TreeType.simple,
+          RadioGroup<TreeType>(
             groupValue: _currentExample,
             onChanged: (TreeType? value) {
-              setState(() {
-                _currentExample = value!;
-              });
+              if (value == null) return;
+              setState(() => _currentExample = value);
             },
+            child: Row(
+              children: <Widget>[
+                Radio<TreeType>(value: TreeType.simple),
+                const Text('Simple'),
+                _spacer,
+                Radio<TreeType>(value: TreeType.custom),
+                const Text('Custom'),
+              ],
+            ),
           ),
-          const Text('Simple'),
-          _spacer,
-          Radio<TreeType>(
-            value: TreeType.custom,
-            groupValue: _currentExample,
-            onChanged: (TreeType? value) {
-              setState(() {
-                _currentExample = value!;
-              });
-            },
-          ),
-          const Text('Custom'),
           const Spacer(),
         ],
       ),