Track the user's current quest

The exit edges of dialog files can now have an extra field, the name of the quest that should start if the user clicks it. That changes the `currentQuest` variable of `SetupLevel.cs`, which causes only the quest arrows for that quest to show on the map and above the gnomes' heads.

I haven't modified any of the dialog files to add that field because I'm not familiar enough with all the things gnomes say.