Add an Onglets (Tabs) widget

New widget where each tab is a real "conteneur" element posed as a child
(see screens/elements/add_tab.py) — this reuses everything that already
exists for a normal container (adding a Répéteur/Conteneur/etc. inside via
"Ajouter DANS ce conteneur", renaming to change the tab's visible label,
deleting via the standard trash icon) instead of inventing a separate
storage format for tabs.

The widget's own properties panel gets a dedicated "Onglets" section to
add a tab, rename one, jump to its content, or delete it. Rendering
(render_onglets.py) builds a tab bar + one panel per tab, switched
client-side (forgeShowTab, in both screen_edit.html and play.html) with
only one panel visible at a time.

Distinct from the existing "activer_onglet" flow action (2.3, manual
show-one/hide-siblings) — that stays available for custom show/hide
wiring; this widget is the turnkey version with tab management built in.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

screens/__init__.py

@@ -73,6 +73,7 @@ from .screens_repo.set_screen_aspect import set_screen_aspect
 from .elements.list_elements import list_elements
 from .elements.get_element import get_element
 from .elements.add_element import add_element
+from .elements.add_tab import add_tab
 from .elements.update_element_geometry import update_element_geometry
 from .elements.save_element_controls import save_element_controls
 from .elements.delete_element import delete_element