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

include/wx/msw/appprogress.h

@@ -21,6 +21,8 @@ public:
     wxAppProgressIndicator(wxWindow* parent = NULL, int maxValue = 100);
     virtual ~wxAppProgressIndicator();
 
+    virtual bool IsAvailable() const wxOVERRIDE;
+
     virtual void SetValue(int value) wxOVERRIDE;
     virtual void SetRange(int range) wxOVERRIDE;
     virtual void Pulse() wxOVERRIDE;