Adding `on_activate` event on process, to notify when it get activated.
For instance, if a user opens the route_a which depends on route_b, and
after that he goes back to route_a, route_a won't render again, but a
series of events must to performed. This is where the `on_activate`
comes in. When a process is running or when a view is already rendered,
and another route asks for it, it'll pop out this event.