Add Cocoa implementation of wxStandardPaths

Previously wxStandardPathsCF was used for all Mac builds and it used
FSFindFolder() (CoreFoundation) calls in Carbon builds, but many hard coded
values in Cocoa builds. The Cocoa implementation uses NSFileManager and
NSBundle to retrieve the folder locations from the system.

Closes https://github.com/wxWidgets/wxWidgets/pull/89

docs/changes.txt

@@ -186,3 +186,4 @@ wxOSX/Cocoa:
 - Add support for wxEVT_COMBOBOX_DROPDOWN and wxEVT_COMBOBOX_CLOSEUP
   events (Igor Korot).
 - Implement strike-through support in wxFont (Igor Korot).
+- Provide native implementation of wxStandardPaths (Tobias Taschner).