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.

interface/wx/gdicmn.h

@@ -1256,6 +1256,10 @@ wxRect wxGetClientDisplayRect();
     The @c x component of the returned wxSize object contains the horizontal
     resolution and the @c y one -- the vertical resolution.
 
+    @note Use of this function is not recommended in the new code as it only
+        works for the primary display. Use wxDisplay::GetPPI() to retrieve
+        the resolution of the appropriate display instead.
+
     @header{wx/gdicmn.h}
 
     @see wxDisplay