Use FSEvents in wxFileSystemWatcher on OS X

The FSEvents API allows for creating watches in entire trees of
directories in an efficient manner.

Closes #16969.

docs/changes.txt

@@ -161,6 +161,7 @@ wxMSW:
 
 wxOSX/Cocoa:
 
+- Use more efficient FSEvents (10.7+) in wxFileSystemWatcher (Roberto Perpuly).
 - Implement wxWindow::Disable() for non-native controls too (Steve Browne).
 - Fix wxEVT_CHAR for non-BMP Unicode characters (ARATA Mizuki).
 - Add support for wxEVT_COMBOBOX_DROPDOWN and wxEVT_COMBOBOX_CLOSEUP