Describe Move toward and Distance in world units (#296)

- Behaviors: Distance measures between two objects in world units,
  however deep each one is nested.
- Behaviors: new action Move toward, with its inputs, its outputs and
  how it moves below a moved or turned parent.

Behaviors.md

@@ -224,13 +224,13 @@ playing use the same two words.
   struct with exactly that struct asset's fields and reports anything else in the console, see
   [Structs](Structs)), `Sub list`,
   `Join lists`, `Index of` (by identity where the items are objects), structs `Pack`/`Expand`,
-  `Length`, `Distance`, `Normalize`, `Axis input` (four keys → a
+  `Length`, `Distance` (between two objects in world units, however deep each one is nested), `Normalize`, `Axis input` (four keys → a
   normalized direction, so diagonals are not faster), `Compare` (two numbers count as equal up to
   float noise, a relative 1e-9, so `time - late == 6` holds without a `Round`; `Equals` does the
   same for two numbers, and for a number against a typed text that is a number, like `130`), `And/Or/Not`, `Key is down`,
   `Key was pressed` and `Key was released` (true for the one frame in which the key went down or came up, the frame `On key down` and `On key up` fire in; a key that stays held is not pressed again),
   `Mouse position`, `Mouse button`, `Is hovered`, `Is touching`, `Layer of` (the collision layer of an object by name and number, see [Colliders](Colliders)), `Animation playing` (whether an animation runs on an object, its progress and time left) and `Animation length` (see [Animations](Animations)), `Get velocity`, `Joint position` (how far a hinge or a slider stands from how it was placed, see [Physics](Physics#joints)), `Current state` (name and id), `Is in state` (one boolean per state) ·
-  actions `Move` (dx, dy, scale, dt — connect an event's dt for per-second movement; in the object's own axes, which a turned parent turns too; with *Collide with solids* it slides along what it hits, less friction, and outputs `blocked x/y`, `hit` and the contact `normal x/y`, see [Colliders](Colliders)), `Set position`, `Spawn`
+  actions `Move` (dx, dy, scale, dt — connect an event's dt for per-second movement; in the object's own axes, which a turned parent turns too; with *Collide with solids* it slides along what it hits, less friction, and outputs `blocked x/y`, `hit` and the contact `normal x/y`, see [Colliders](Colliders)), `Move toward` (x, y in world units, speed, dt: steps the object toward that point by at most speed times dt and stops exactly on it, also below a moved or turned parent; with *Collide with solids* it moves the way `Move` does; outputs `arrived`, the `distance` still left and `hit`. The way to walk an enemy to a spot, home or after the player), `Set position`, `Spawn`
   (asset input + x/y/z, optional parent → outputs the new object; the asset is picked on the node or arrives over an edge, from an `Asset` node, a variable or a list item. A pick saved while it was still a setting reads as that input's value, so older projects spawn as before; the same holds for the asset of `Is asset`), `Destroy` (removes the object with everything below it; with **keep children** on, only the object goes: its children move up to its parent (top level when it had none) and keep their place, rotation and size in the world, and the output `children` is the list of them, nearest level only. Its behaviors, its rigidbody and its joints go with it either way. In the editor run, children that are content of a placed container move up for the run only: their place belongs to the template, so the scene does not learn it), `Add joint` (ties an object to another while the game runs: the picked Joint asset is placed on *object* at x/y and connected to *connected*, the scene when nothing arrives; outputs the new joint, which `Destroy` lets go again, see [Physics](Physics#joints)), `Emit` (starts a particle source over, see *Particles*),
   `Play sound` / `Stop sound` (see [Sounds](Sounds): the asset says how loud and whether it loops, the node's
   `volume` multiplies that and its `loop` either leaves it alone or overrules it in either