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

include/wx/app.h

@@ -454,6 +454,10 @@ public:
         //          with _extreme_ care or, better, don't use at all!
     virtual bool Yield(bool onlyIfNeeded = false) = 0;
 
+        // returns true if the main thread is inside a Yield() call
+    bool IsYielding() const
+        { return m_isInsideYield; }
+
         // this virtual function is called in the GUI mode when the application
         // becomes idle and normally just sends wxIdleEvent to all interested
         // parties
@@ -576,6 +580,7 @@ protected:
     // does any of our windows have focus?
     bool m_isActive;
 
+    bool m_isInsideYield;
 
     DECLARE_NO_COPY_CLASS(wxAppBase)
 };