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/statusbr.h

@@ -38,9 +38,7 @@ protected:
     virtual void InitColours() wxOVERRIDE;
 
 private:
-    wxColour m_textActive, m_textInactive,
-             m_bgActiveFrom, m_bgActiveTo,
-             m_borderActive, m_borderInactive;
+    wxColour m_textActive, m_textInactive;
 
     wxDECLARE_DYNAMIC_CLASS(wxStatusBarMac);
     wxDECLARE_EVENT_TABLE();