JSONize gates

[codemime]

Follows #15927. Hard-coded gates no more. We can now modify/expand them freely. The gate code was placed to separate files to de-bloat the rest a tiny bit. Fix: closing gate damages contents of all involved tiles, not only the first damageable. Also, a gate can now be opened by both player's activity and completely by itself (the player witnessing). It would be useful e.g. for quests with restricted areas.

Edit: NPCs could now use gates as well (2D points are replaced with 3D ones according to @Coolthulhu 's suggestion).

[Night-Pryanik]

Great work!
Though ideally I'd prefer to remove "wall type - handle type" dependence and replace it with "gate type - handle type". In that case we wouldn't need to create different gate types for every type of wall and could use one usual mechanical handle for ordinary walls, concrete walls, brick walls etc to operate metal gate, for example.
Something like array of types of walls which we could use with some specific handle.

[codemime]

Thanks. Good idea, makes sense to me. Since multiple levers/poles/ropes already exist to trigger the gates (in existing saved games as well), the handle field should become an array too. So that it will be many-to-many relationship. Usage of the neat generic_factory becomes trickier in that case. Though I think it allows insertion during loading so I can make as many copies as I need (or store pointers).

[Coolthulhu]

Would be better if it used the 3D (tripoint) map::ter rather than the compatibility 2D one. That way NPCs could be made to use it too.

[codemime]

Agree. Good suggestion.

[codemime]

Done.