Replace input to define field visibility

[hvelarde]

Currently, the visibility of a tile's field is defined through a couple of radio buttons labeled Yes and No; we need to replace this with a single checkbox.

I know this will require a refactoring of the configuration infrastructure as discussed on #100, so please evaluate the options; we could open a different ticket to implement that.

I will love to ear what @jpgimenez and @leorochael have to say on this.

[hvelarde]

I found a very simple and clean solution for this here: http://stackoverflow.com/questions/9670819/how-do-i-make-a-boolean-field-required-in-a-z3c-form

[agnogueira]

Solved here only with CSS : https://github.com/collective/collective.cover/tree/tile-config

[hvelarde]

please make a pull request

[agnogueira]

#243