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.

docs/changes.txt

@@ -127,6 +127,7 @@ All (GUI):
 - Allow binding to events generated by their items in submenus too.
 - Add wxGrid::SetCornerLabelValue() (Pavel Kalugin).
 - Add strikethrough support for fonts defined in XRC.
+- Add wxDisplay::GetPPI().
 
 wxGTK: