DEMO JS: Check for errors only after submit
Form Element::notify_change(): Use errors() to get list of errors

- therefore incl errors from 'req=>true'
Form Element *: instead of onchange event use onblur
Form Element: Add div_errors where errors will be shown as list
Form Element (PHP): add form_root, rename parent->form_parent, add form_root.form
fix
Form / Form Element: change behaviour of show_errors()

- no longer prints errors, but rather sets a flag which causes
  the form_element->show() to print errors to its div_errors
Demo: adapt to new show_errors() behaviour