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/core/private.h

@@ -984,6 +984,8 @@ public :
 
     virtual void SetRepresentedFilename(const wxString& WXUNUSED(filename)) { }
 
+    virtual void SetBottomBorderThickness(int WXUNUSED(thickness)) { }
+
 #if wxOSX_USE_IPHONE
     virtual CGFloat GetWindowLevel() const { return 0.0; }
 #else