Add wxDisplay::GetPPI() to use instead of wxGetDisplayPPI()

While this is not done for all the ports yet, the new API allows
returning different PPI values for different monitors, unlike the old
(and still existing, but implemented in terms of the new one) global
function.

include/wx/display.h

@@ -11,6 +11,7 @@
 #define _WX_DISPLAY_H_BASE_
 
 #include "wx/defs.h"
+#include "wx/gdicmn.h"      // wxSize
 
 // NB: no #if wxUSE_DISPLAY here, the display geometry part of this class (but
 //     not the video mode stuff) is always available but if wxUSE_DISPLAY == 0
@@ -77,6 +78,9 @@ public:
     // get the depth, i.e. number of bits per pixel (0 if unknown)
     int GetDepth() const;
 
+    // get the resolution of this monitor in pixels per inch
+    wxSize GetPPI() const;
+
     // name may be empty
     wxString GetName() const;