[Material] Update slider gallery demo, including range slider (#34597)

Update the flutter_gallery Slider demo by aligning the text field in the first slider, adding range slider examples (including custom range thumbs), and separating single slider and range slider examples with tabs.

examples/flutter_gallery/lib/gallery/demo.dart

@@ -46,11 +46,15 @@ class TabbedComponentDemoScaffold extends StatelessWidget {
     this.title,
     this.demos,
     this.actions,
+    this.isScrollable = true,
+    this.showExampleCodeAction = true,
   });
 
   final List<ComponentDemoTabData> demos;
   final String title;
   final List<Widget> actions;
+  final bool isScrollable;
+  final bool showExampleCodeAction;
 
   void _showExampleCode(BuildContext context) {
     final String tag = demos[DefaultTabController.of(context).index].exampleCodeTag;
@@ -93,29 +97,28 @@ class TabbedComponentDemoScaffold extends StatelessWidget {
       child: Scaffold(
         appBar: AppBar(
           title: Text(title),
-          actions: (actions ?? <Widget>[])..addAll(
-            <Widget>[
-              Builder(
-                builder: (BuildContext context) {
-                  return IconButton(
-                    icon: const Icon(Icons.library_books, semanticLabel: 'Show documentation'),
-                    onPressed: () => _showApiDocumentation(context),
-                  );
-                },
-              ),
-              Builder(
-                builder: (BuildContext context) {
-                  return IconButton(
-                    icon: const Icon(Icons.code),
-                    tooltip: 'Show example code',
-                    onPressed: () => _showExampleCode(context),
-                  );
-                },
-              ),
-            ],
-          ),
+          actions: (actions ?? <Widget>[])..add(
+            Builder(
+              builder: (BuildContext context) {
+                return IconButton(
+                  icon: const Icon(Icons.library_books, semanticLabel: 'Show documentation'),
+                  onPressed: () => _showApiDocumentation(context),
+                );
+              },
+            ),
+          )..addAll(showExampleCodeAction ? <Widget>[
+            Builder(
+              builder: (BuildContext context) {
+                return IconButton(
+                  icon: const Icon(Icons.code),
+                  tooltip: 'Show example code',
+                  onPressed: () => _showExampleCode(context),
+                );
+              },
+            ),
+          ] : <Widget>[]),
           bottom: TabBar(
-            isScrollable: true,
+            isScrollable: isScrollable,
             tabs: demos.map<Widget>((ComponentDemoTabData data) => Tab(text: data.tabName)).toList(),
           ),
         ),