[validatecontroller] allow additional args to be passed to the js callback

[javascript] make inlinedform-added event bindable $(form).trigger('inlinedform-added') was not usable since form had no existence outside the inner callback. Use the general CubicWeb object to trigger the event so it's easy to bind a callback wherever it's needed.

[forms] take a look at inlined form to guess whether the form requires multipart or not