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/GUI_Utils.hpp

@@ -95,6 +95,7 @@ public:
 #ifndef __WXOSX__ // Don't call SetFont under OSX to avoid name cutting in ObjectList 
         this->SetFont(m_normal_font);
 #endif
+        this->CenterOnParent();
 
         // Linux specific issue : get_dpi_for_window(this) still doesn't responce to the Display's scale in new wxWidgets(3.1.3).
         // So, calculate the m_em_unit value from the font size, as before