forms: add list-add class to "add item" widget (#45688)

tests/test_form_pages.py

@@ -8655,6 +8655,7 @@ def test_block_repeated(pub, blocks_feature):
     assert resp.text.count('>Test<') == 1
     assert resp.text.count('>hintblock<') == 1
     assert 'Add another' in resp
+    assert resp.html.find('div', {'class': 'list-add'})
     resp = resp.form.submit('f1$add_element')
     assert resp.text.count('>Test<') == 2
     assert resp.text.count('>hintblock<') == 1

wcs/qommon/form.py

@@ -237,6 +237,10 @@ TextWidget.render_content = text_render_content
 
 
 class SubmitWidget(quixote.form.widget.SubmitWidget):
+    def __init__(self, *args, **kwargs):
+        self.extra_css_class = kwargs.pop('extra_css_class', None)
+        super().__init__(*args, **kwargs)
+
     def render_content(self):
         if self.name in ('cancel', 'previous', 'save-draft'):
             self.attrs['formnovalidate'] = 'formnovalidate'
@@ -387,7 +391,7 @@ class Form(QuixoteForm):
         r = TemplateIO(html=True)
         classnames = '%s widget %s-button %s' % (
                 button.__class__.__name__, button.name,
-                getattr(button, 'extra_css_class', ''))
+                getattr(button, 'extra_css_class', None) or '')
         r += htmltext('<div class="%s">') % classnames
         r += htmltext('<div class="content">')
         r += button.render_content()