Refactor navigator event handling

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.

Change-Id: I29d4a082cb01ff7943b7822ea9aa8ad622fda593
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>