Check for task dialog availability even under Vista and later.

Task dialogs are not always available under Vista and later Windows versions
because we might not be using the right (i.e. 6+) version of comctl32.dll.

Improve the check for task dialog availability and fall back to the classic
message box if we can't use it.

Closes #12553.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65816 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/msgdlg.h

@@ -59,8 +59,6 @@ private:
     // aren't available.
     int ShowMessageBox();
 
-    // used by ShowModal() to display a task dialog.
-    int ShowTaskDialog();
 
     WXHANDLE m_hook; // HHOOK used to position the message box