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/gdicmn.h

@@ -1080,6 +1080,9 @@ extern WXDLLIMPEXP_DATA_CORE(const wxPoint) wxDefaultPosition;
 extern void WXDLLIMPEXP_CORE wxInitializeStockLists();
 extern void WXDLLIMPEXP_CORE wxDeleteStockLists();
 
+// Note: all the display-related functions here exist for compatibility only,
+// please use wxDisplay class in the new code
+
 // is the display colour (or monochrome)?
 extern bool WXDLLIMPEXP_CORE wxColourDisplay();