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/cocoa/private.h

@@ -329,6 +329,8 @@ public :
 
     virtual void SetRepresentedFilename(const wxString& filename) wxOVERRIDE;
 
+    virtual void SetBottomBorderThickness(int thickness) wxOVERRIDE;
+
     wxNonOwnedWindow*   GetWXPeer() { return m_wxPeer; }
 
     CGWindowLevel   GetWindowLevel() const wxOVERRIDE { return m_macWindowLevel; }

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

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();

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();