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

samples/dialogs/dialogs.h

@@ -430,6 +430,7 @@ public:
 #if wxUSE_PROGRESSDLG
     void ShowProgress(wxCommandEvent& event);
 #endif // wxUSE_PROGRESSDLG
+    void ShowAppProgress(wxCommandEvent& event);
 
 #if wxUSE_ABOUTDLG
     void ShowSimpleAboutDialog(wxCommandEvent& event);
@@ -570,6 +571,7 @@ enum
     DIALOGS_ONTOP,
     DIALOGS_MODELESS_BTN,
     DIALOGS_PROGRESS,
+    DIALOGS_APP_PROGRESS,
     DIALOGS_ABOUTDLG_SIMPLE,
     DIALOGS_ABOUTDLG_FANCY,
     DIALOGS_ABOUTDLG_FULL,