Allow quests to set a map as a target step

[snipercup]

The quests have limited variety in steps. Targeting a map is not one of them.

We should change this to:

• Have the quest editor accept a map as the target for a step
• When the player reaches this step in the quest, the player will need to move to that map
• The overmap should then put a marker on the nearest instance of that map
• When the player enters that instance of the map, the quest step is completed

For example:

• Have the quest step be a forest map as a target
• The overmap marks the nearest forest tile as the target for the current quest
• The player walks to the target map and completes the step.