Refactor navigator event handling
Separate PPS based navigator event notification handling and implementation
of the actual event handling.
Like the similar refactoring for screen events, this will allow to reuse the
same event handler class for BPS driven event processing.
Backport of 4fac40f8b0e7b2a6f5f41eb82196e8b197bc851a
Reviewed-by: Sean Harmer <email@example.com>
Reviewed-by: Rafael Roquetto <firstname.lastname@example.org>
Reviewed-by: Robin Burchell <email@example.com>