Validation rules

[almet]

I've opened a series of issues more to start a discussion about certain features than to tell it should be done. This is one of them and we need to decide if we want to have it or not.

Validation rules are already in the center of the project, but there is currently no real user control for them. Maybe we should add a way to have advanced control on the required fields. I'm not really sure how other projects do it, but that could be interesting.