Render statusbar natively on macOS

Instead of trying to mimic native statusbar background rendering in our
own code, use NSWindow setContentBorderThickness:forEdge: to extend the
border that is drawn by the system. This is the canonical way of doing
statusbars ("bottom bar") in AppKit.

The text is still drawn generically, but that's a much smaller issue
than trying to match the border gradient.

include/wx/osx/frame.h

@@ -71,6 +71,8 @@ public:
                                            long style = wxSTB_DEFAULT_STYLE,
                                            wxWindowID id = 0,
                                            const wxString& name = wxASCII_STR(wxStatusLineNameStr)) wxOVERRIDE;
+
+    virtual void SetStatusBar(wxStatusBar *statbar) wxOVERRIDE;
 #endif // wxUSE_STATUSBAR
 
     void PositionBars();