Add labels to the slider demos (#5639)

Previously, it was unclear that the middle slider was disabled.

Fixes #5517

examples/flutter_gallery/lib/demo/slider_demo.dart

@@ -22,8 +22,10 @@ class _SliderDemoState extends State<SliderDemo> {
       body: new Column(
         mainAxisAlignment: MainAxisAlignment.spaceAround,
         children: <Widget>[
-          new Center(
-            child: new Slider(
+          new Column(
+            mainAxisSize: MainAxisSize.min,
+            children: <Widget> [
+              new Slider(
                 value: _value,
                 min: 0.0,
                 max: 100.0,
@@ -32,11 +34,21 @@ class _SliderDemoState extends State<SliderDemo> {
                     _value = value;
                   });
                 }
-            )
               ),
-          new Center(child: new Slider(value: 0.25, onChanged: null)),
-          new Center(
-            child: new Slider(
+              new Text('Continuous'),
+            ]
+          ),
+          new Column(
+            mainAxisSize: MainAxisSize.min,
+            children: <Widget> [
+              new Slider(value: 0.25, onChanged: null),
+              new Text('Disabled'),
+            ]
+          ),
+          new Column(
+            mainAxisSize: MainAxisSize.min,
+            children: <Widget> [
+              new Slider(
                 value: _discreteValue,
                 min: 0.0,
                 max: 100.0,
@@ -47,7 +59,9 @@ class _SliderDemoState extends State<SliderDemo> {
                     _discreteValue = value;
                   });
                 }
-            )
+              ),
+              new Text('Discrete'),
+            ]
           ),
         ]
       )