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/appprogress.h

@@ -17,6 +17,9 @@ class WXDLLIMPEXP_CORE wxAppProgressIndicatorBase
 public:
     wxAppProgressIndicatorBase() {}
     virtual ~wxAppProgressIndicatorBase() {}
+
+    virtual bool IsAvailable() const = 0;
+
     virtual void SetValue(int value) = 0;
     virtual void SetRange(int range) = 0;
     virtual void Pulse() = 0;
@@ -28,6 +31,22 @@ private:
 
 #if defined(__WXMSW__)
     #include "wx/msw/appprogress.h"
+#else
+    class wxAppProgressIndicator : public wxAppProgressIndicatorBase
+    {
+    public:
+        wxAppProgressIndicator(wxWindow* WXUNUSED(parent) = NULL,
+                               int WXUNUSED(maxValue) = 100)
+        {
+        }
+
+        virtual bool IsAvailable() const wxOVERRIDE { return false; }
+
+        virtual void SetValue(int WXUNUSED(value)) wxOVERRIDE { }
+        virtual void SetRange(int WXUNUSED(range)) wxOVERRIDE { }
+        virtual void Pulse() wxOVERRIDE { }
+        virtual void Reset() wxOVERRIDE { }
+    };
 #endif
 
 #endif  // _WX_APPPROG_H_