Implement wxDisplaySize() and wxClientDisplayRect() via wxDisplay

Instead of forwarding to these functions from wxDisplay implementation
in wxUSE_DISPLAY==0 case, make the functions themselves wrappers around
wxDisplay, which may, or not, depending on the platform, have a simpler
implementation in wxUSE_DISPLAY==0 case, but is always available in any
case.

As part of this change, only use src/osx/core/display.cpp in macOS
builds, not iOS ones and update the Xcode project accordingly too.

This cuts down on code duplication, especially in wxGTK, and facilitates
further additions to wxDisplay API.

include/wx/private/display.h

@@ -10,6 +10,7 @@
 #ifndef _WX_PRIVATE_DISPLAY_H_
 #define _WX_PRIVATE_DISPLAY_H_
 
+#include "wx/display.h"
 #include "wx/gdicmn.h"      // for wxRect
 #include "wx/vector.h"
 
@@ -112,18 +113,63 @@ protected:
     wxDECLARE_NO_COPY_CLASS(wxDisplayImpl);
 };
 
+// ----------------------------------------------------------------------------
+// wxDisplayImplSingle: the simplest possible impl for the main display only
+// ----------------------------------------------------------------------------
+
+// Note that this is still an ABC and GetGeometry() and GetClientArea() methods
+// must be implemented in the derived classes.
+
+class WXDLLEXPORT wxDisplayImplSingle : public wxDisplayImpl
+{
+public:
+    wxDisplayImplSingle() : wxDisplayImpl(0) { }
+
+    virtual wxString GetName() const wxOVERRIDE { return wxString(); }
+
+#if wxUSE_DISPLAY
+    // no video modes support for us, provide just the stubs
+    virtual wxArrayVideoModes
+    GetModes(const wxVideoMode& WXUNUSED(mode)) const wxOVERRIDE
+    {
+        return wxArrayVideoModes();
+    }
+
+    virtual wxVideoMode GetCurrentMode() const wxOVERRIDE
+    {
+        return wxVideoMode();
+    }
+
+    virtual bool ChangeMode(const wxVideoMode& WXUNUSED(mode)) wxOVERRIDE
+    {
+        return false;
+    }
+#endif // wxUSE_DISPLAY
+
+    wxDECLARE_NO_COPY_CLASS(wxDisplayImplSingle);
+};
+
 // ----------------------------------------------------------------------------
 // wxDisplayFactorySingle
 // ----------------------------------------------------------------------------
 
-// this is a stub implementation using single/main display only, it is
-// available even if wxUSE_DISPLAY == 0
+// This is the simplest implementation of wxDisplayFactory using single/main
+// display only. It is used when wxUSE_DISPLAY == 0 because getting the size of
+// the main display is always needed.
+//
+// Note that this is still an ABC and derived classes must implement
+// CreateSingleDisplay().
+
 class WXDLLIMPEXP_CORE wxDisplayFactorySingle : public wxDisplayFactory
 {
 public:
-    virtual wxDisplayImpl *CreateDisplay(unsigned n) wxOVERRIDE;
     virtual unsigned GetCount() wxOVERRIDE { return 1; }
     virtual int GetFromPoint(const wxPoint& pt) wxOVERRIDE;
+
+protected:
+    virtual wxDisplayImpl *CreateDisplay(unsigned n) wxOVERRIDE;
+
+    virtual wxDisplayImpl *CreateSingleDisplay() = 0;
 };
 
 #endif // _WX_PRIVATE_DISPLAY_H_