Add new events on_map_finished and on_opening_transition_finished

[Diarandor]

Add missing transition events to make our lives easier:

Item events:

• item:on_map_finished(map)
• item:on_opening_transition_finished(map, destination)

Menu events:

• menu:on_map_finished(map)
• menu:on_map_changed(map)
• menu:on_opening_transition_finished(destination, map)

Game events:

• game:on_map_finished(map)
• game:on_opening_transition_finished(map, destination)

[Diarandor]

I'd like to use "item:on_map_finished(map)" for some items like the custom shield, to destroy the custom entity shield before the opening transition starts, because the shield remains visible in the original map during this transition (which is bad). I know there are dirtier workarounds like registering this with the multi_events script, but I'd prefer to use a cleaner solution with a normal event. These events are gonna be very useful for other items too.

[Diarandor]

This issue can wait for v1.7 since we will be using the multi_event script when necessary.