Make wxAppProgressIndicator actually usable.

First of all, do define it under non-MSW platforms.

Second, don't crash in it when running under XP where wxTaskBarButton is not
available.

Also add IsAvailable() method to check for its availability explicitly and add
a demonstration of this class to the dialogs sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77706 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/appprogress.h

@@ -40,6 +40,17 @@ public:
     */
     virtual ~wxAppProgressIndicator();
 
+    /**
+        Check if the application progress display is available.
+
+        Currently this only returns @true when using wxMSW and running under
+        Vista or later system, which provide task bar button API.
+
+        If this method returns @false, no other methods of this class do
+        anything, but they may still be called without any ill effects.
+     */
+    bool IsAvailable() const;
+
     /**
         Set the progress value in taskbar button of parent window.