Move wxBookCtrlBase::WXHandleDPIChanged() to wxWithImages itself

This function is not really wxBookCtrl-specific and can be used with
other classes as well, so allow reusing it in them.

No real changes, this is just a refactoring.

include/wx/bookctrl.h

@@ -360,10 +360,6 @@ private:
     // common part of all ctors
     void Init();
 
-    // update the images if we use them on DPI change
-    void WXHandleDPIChanged(wxDPIChangedEvent& event);
-
-
     // internal border
     unsigned int m_internalBorder;