jforms : support of a cancel button

[laurentj]

A special submit button could be used, to cancel a form : it submits the form but jForms::fill could check if this is the cancel button to know if it should get or not all values. jFormsBase could have a method, isCancelled to know if the user has cancelled or not and so we can doing things in the controller like doing a redirection.

The button cancel could be declared in <submit> element, with a <item value="cancel">.

[laurentj]

Changes should be made also in jforms.js to avoid to do validation on controls.

[bibo]

At first glance, isn't it simpler for anyone to add a link beside submit button. the redirection is evident here and no need to submit, etc. ? the only drawback I see at the moment, is that using a link and not applying style to submit button, will make them display really differently. That is a pain but style can arrange this.