Add wxWindow::WXNotifyDPIChange() for wxOSX and wxGTK

This refactors the already existing code in wxOSX to make it available
in the common class and allow reusing it in wxGTK.

No changes yet.

include/wx/osx/window.h

@@ -151,7 +151,6 @@ public:
     // --------------
 
     void OnMouseEvent( wxMouseEvent &event );
-    void OnDPIChanged( wxDPIChangedEvent& event );
 
     void MacOnScroll( wxScrollEvent&event );
 

include/wx/window.h

@@ -1675,6 +1675,10 @@ public:
 #ifdef wxHAS_DPI_INDEPENDENT_PIXELS
     // Return the DPI corresponding to the given scale factor.
     static wxSize MakeDPIFromScaleFactor(double scaleFactor);
+
+    // Notify all non-top-level children of the given (typically top-level
+    // itself) window about the DPI change.
+    void WXNotifyDPIChange(double oldScaleFactor, double newScaleFactor);
 #endif // wxHAS_DPI_INDEPENDENT_PIXELS
 
 protected: