Allow to run for loops over collections of items

[gggeek]

We now have a 'for' loop, as well as matchers that allow selecting N items.

It should thus not be too complicated to implement a way to apply a mig. step to each element in a set.

Ex usecase: "add a child content to all the children of node X"

Go foreach !

[gggeek]

PS: this should come close to supporting most of the usecases of PR #81

[gggeek]

Done in release 5.4