Rewire hover as a flow trigger

Add "survol"/"fin_survol" trigger events (mouseenter/mouseleave,
same anti-doublon pattern as bindClicks()) so hovering an element can
run flow nodes, instead of the old static hover-text-only control
removed from the properties panel. Also add "Contenu" as a settable
"Modifier un élément" property so an action can display a defined
text on another element on hover — the concrete use case that
motivated this.

Verified live with Playwright: a "Au survol" trigger on one element
correctly updates another element's text via the action, and text
stays put with no "Fin du survol" wired (explicit, no implicit
revert, consistent with "Au clic").

screens/labels/trigger_labels.py

@@ -2,4 +2,6 @@ TRIGGER_LABELS = {
     "clic": "Au clic",
     "soumission": "À la soumission",
     "affichage": "À l'affichage de l'écran",
+    "survol": "Au survol",
+    "fin_survol": "Fin du survol",
 }