[gimp] app: fix dragging color boxes into item dockables.

[Jehan <jehanp src gnome org>]

commit 78d9f9799f79d3e1979c04c7e28fdee7721f2638
Author: Jehan <jehan girinstud io>
Date:   Sun Sep 26 17:06:57 2021 +0200

    app: fix dragging color boxes into item dockables.
    
    Commit 83b3d9e5 broke dragging color widgets into channels dockables or
    other item dockables.
    Thanks to Massimo for raising that these lines were needed for this.

 app/widgets/gimpcontainertreeview-dnd.c | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/app/widgets/gimpcontainertreeview-dnd.c b/app/widgets/gimpcontainertreeview-dnd.c
index 81123f8165..37aec76ebf 100644
--- a/app/widgets/gimpcontainertreeview-dnd.c
+++ b/app/widgets/gimpcontainertreeview-dnd.c
@@ -540,6 +540,13 @@ gimp_container_tree_view_drag_drop (GtkWidget             *widget,
           gtk_drag_finish (context, success, FALSE, time);
           g_list_free (src_viewables);
         }
+      else
+        {
+          /* Necessary for instance for dragging color components onto
+           * item dialogs.
+           */
+          gtk_drag_get_data (widget, context, target, time);
+        }
 
       return TRUE;
     }