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>

routes/elements/__init__.py

@@ -6,4 +6,5 @@ from . import (
     element_delete,
     element_duplicate,
     element_rename,
+    element_tab_add,
 )