Allow form fields required state to be controlled with a function

[mpociot]

This PR allows you to use a callback function with model to control the required state of a form field.

Example to make a field only required, if another field is empty:

{
    type: "text",
    label: "Phone mobile",
    model: "phone_mobile",
    required: function(model) {
    	return (model.phone_home === null);
    }
}

[icebob]

Good job! Thanks!

[mpociot]

What do you think of having the ability to use callback functions for the other boolean checks as well? I would provide a callback for this too.

[icebob]

Please tell which properties do you think?

[mpociot]

I was thinking of readonly and featured. Then all Boolean properties have the same behavior.

[icebob]

Ok, good idea.

[icebob]

@mpociot could you make a new PR with readonly and featured?