Target field in entities

A "target" field has been added to effects to define which entities they have to be applied to.

There are three options:
- **_this**. Applies the effect only to the entity that contains the effect. This is the default option if target is not specified. 
- **all**. Applies the effect to all the entities found.
- **each _target {expression}**. Applies the effect to each entity that matches the expression given among curly brackets. In this expression it is possible to refer to the entity's properties using special variable ```$_target```. ```$_this``` can be used as well to refer to the effect's owner entity, although is not of much use now.