Set entity property effect

This PR adds a new effect that allows setting whatever property in a given entity through Accessor. It takes two arguments:

- The property to be set, identified following the accessor namespace,
- An expression that determines the new value.

Example:
```{
         class: changeentityproperty,
         property: components<velocity>.x,
         expression: f30
```}

will get the "velocity" component of the target entity and set its velocity's x value to 30.

Three first commits are related to enhancing Accessor so it can actually modify properties,
All the others are the effect itself