Decorator Targets

[vilicvane]

Decorator target is one of the successful concept in Drop v0.1, but it could be more powerful in v0.2.

In v0.1, decorator target is a collection of some elements plus the boundaries. I am thinking about making it capable of containing sub decorator targets directly. So that we may make modifiers like each a decorator target containing sub targets.

In v0.1, a modifier has to be the first decorator of a specific decorator target, this is not a big deal. But since I am thinking about removing the type marker #, it might be better if we can also remove that limit and make the differences of modifier and processor hidden from the users.