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/Tab.cpp

@@ -3338,7 +3338,7 @@ void Tab::save_preset(std::string name /*= ""*/, bool detach)
 //!	m_treectrl->OnSetFocus();
 
     if (name.empty()) {
-        SavePresetDialog dlg(m_type, detach ? _u8L("Detached") : "");
+        SavePresetDialog dlg(m_parent, m_type, detach ? _u8L("Detached") : "");
         if (dlg.ShowModal() != wxID_OK)
             return;
         name = dlg.get_name();