[js utils] backport some generic code from comments cube
to handle inline ajax form as you get to add comment to entities.
Following stuff has been generalized and backported to ease such things:
* lazy_view_holder() method on EntityCtxComponent class, to build
place holder where the form will be inserted
* ajax_composite_form() function in cw.web.views.ajaxedit, to build
the form itself
* reload() and reloadCtxComponentsSection() javascript function in
cubicweb.ajax.js for the javascript processing side