Integrate event parsing into NPC dialogs

This allows for event execution when a conversation starts, as well as after each line of dialog. It's also possible to have no dialog at all and only events. Redundant event parsing from the NPC class has been moved to the EventManager.

To demonstrate usage, here's an example [dialog] section with some events:
```
[dialog]
topic=Event test

# This gets executed when initiating conversation
reward_currency=100

# Start off with some dialog
him=Thanks for taking care of the goblins. Don't spend all that gold in one place.
him=You look hurt. Let me heal your wounds.

# This is executed when the player advances the dialog
restore=all

# More dialog
him=Now get out of here.

# And one last event. Again, executed when advancing the dialog
intermap=mineshaft_longsword.txt,44,61
soundfx=soundfx/powers/teleport.ogg
```

Because of its new multi-purpose use, I've also renamed the `Map_Event` class to `Event`.