Add minimal DPI change handler to wxGrid

This handler redoes wxGrid layout and refreshes it to at least avoid
ugly display artifacts when moving wxGrid window between displays with
different DPI.

include/wx/generic/grid.h

@@ -2415,6 +2415,13 @@ protected:
     friend class wxGridHeaderCtrl;
 
 private:
+    // This is called from both Create() and OnDPIChanged() to (re)initialize
+    // the values in pixels, which depend on the current DPI.
+    void InitPixelFields();
+
+    // Event handler for DPI change event recomputes pixel values and relays
+    // out the grid.
+    void OnDPIChanged(wxDPIChangedEvent& event);
 
     // implement wxScrolledCanvas method to return m_gridWin size
     virtual wxSize GetSizeAvailableForScrollTarget(const wxSize& size) wxOVERRIDE;