use a common m_isInsideYield flag instead of static booleans in all ports; add a IsYielding() test which can help to fix unwanted re-entrancies

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@57637 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/appcmn.cpp

@@ -78,6 +78,8 @@ wxAppBase::wxAppBase()
 
     m_isActive = true;
 
+    m_isInsideYield = false;
+
     // We don't want to exit the app if the user code shows a dialog from its
     // OnInit() -- but this is what would happen if we set m_exitOnFrameDelete
     // to Yes initially as this dialog would be the last top level window.