Better control of what is an undoable interaction

[ClementSparrow]

Related to #23.

PuzzleScript has a "no undo to same state" policy: if after undoing, the level is in the same state than it was before the undo, then the engine keeps on undoing until it reaches a different state.

In addition to avoiding confusion, this policy allows avoiding undos for meaningless inputs like bumping into a wall or kicking a rock. And that's definitely interactions that we don't want to put in the undo stack. We want the players to be able to explore interacting with the world and still be able to backtrack quickly.

The problem is that the "no undo to same state" is probably not the best way to implement that. For instance, if you have elements in the level that use a randomized variant at each frame to make an animation (e.g., smoke, flames, wind in the grass, butterflies...) then the state will be different and bumping into a wall will be pushed to the undo stack (worse: imagine you have levels with such random elements and levels without, then the behavior of undo after bumping into a wall depends on the level).

Another example: if you have an avatar with four possible orientations, and bumping into a wall makes it turn to face the wall, then again you have a different state before and after bumping into the wall.

So, we probably need a better way to control what is an undoable interaction or what is a "same state".

[m-ender]

Dario's fork has a nosave command which simply doesn't add the current turn to the undo stack. That seems like a good baseline functionality to have.

[ClementSparrow]

I have implemented nosave in commit 63761a3, I forgot it was mentioned in this issue. But it does not fix completely the issue.

I guess that in addition to nosave, we need ways to tell "objects in property XXX should be considered equal for the detection of change" (i.e., if one object in that property is replaced with another object in that property, then it's not a difference). Or "Ignore this object/property in the detection of change" (this will in addition ignore differences when an object in that property is created or destroyed).