Fix #111715: VSE: "Bounding Box Center" pivot point is the same as "Median Point"

The logic to calculate the bounds was missing.

Also add a notifier with `NC_SPACE | ND_SPACE_SEQUENCER` to update the
space.

source/blender/editors/transform/transform_gizmo_2d.cc

@@ -396,6 +396,9 @@ static bool gizmo2d_calc_transform_pivot(const bContext *C, float r_pivot[2])
       has_select = SEQ_collection_len(strips) != 0;
       SEQ_collection_free(strips);
     }
+    else if (pivot_point == V3D_AROUND_CENTER_BOUNDS) {
+      has_select = gizmo2d_calc_bounds(C, r_pivot, nullptr, nullptr);
+    }
     else {
       has_select = seq_get_strip_pivot_median(scene, r_pivot);
     }

source/blender/makesrna/intern/rna_scene.cc

@@ -3951,6 +3951,7 @@ static void rna_def_sequencer_tool_settings(BlenderRNA *brna)
   prop = RNA_def_property(srna, "pivot_point", PROP_ENUM, PROP_NONE);
   RNA_def_property_enum_items(prop, pivot_points);
   RNA_def_property_ui_text(prop, "Pivot Point", "Rotation or scaling pivot point");
+  RNA_def_property_update(prop, NC_SPACE | ND_SPACE_SEQUENCER, nullptr);
 }
 
 static void rna_def_unified_paint_settings(BlenderRNA *brna)