Fixed a Dialog position under OSX

Always set parent for the Dialog, if we want to see the Dialog in the same display as a parent.
Dialog will be shown in the primary monitor otherwise.

+ fixed a draw of the canvas legend when we move application between Retina and non-Retina displays

src/slic3r/GUI/PhysicalPrinterDialog.hpp

@@ -82,7 +82,7 @@ class PhysicalPrinterDialog : public DPIDialog
     void AddPreset(wxEvent& event);
 
 public:
-    PhysicalPrinterDialog(wxString printer_name);
+    PhysicalPrinterDialog(wxWindow* parent, wxString printer_name);
     ~PhysicalPrinterDialog();
 
     void        update();