Add drag-and-drop reordering of sibling elements in the tree panel

Elements can now be reordered within the same container by dragging a row above or below another in the left-hand element tree.

routes/elements/__init__.py

@@ -2,6 +2,7 @@ from . import (
     element_add,
     element_add_child,
     element_geometry,
+    element_reorder,
     element_save,
     element_delete,
     element_duplicate,

routes/elements/element_reorder.py

@@ -0,0 +1,22 @@
+from flask import request, jsonify, abort
+
+import screens
+
+from core.flask_app import app
+
+
+@app.route("/game/<slug>/elements/<int:element_id>/reorder", methods=["POST"])
+def element_reorder(slug, element_id):
+    """Appelé en AJAX quand on glisse un élément au-dessus/en dessous d'un
+    autre DANS LA MÊME ARBORESCENCE (panneau de gauche) pour changer l'ordre
+    des éléments d'un même conteneur — voir reorder_element."""
+    element = screens.get_element(slug, element_id)
+    if not element:
+        abort(404)
+    data = request.get_json(force=True)
+    target_id = int(data["target_id"])
+    position = data.get("position")
+    if position not in ("before", "after"):
+        abort(400)
+    screens.reorder_element(slug, element["screen_id"], element_id, target_id, position)
+    return jsonify({"ok": True})