dnd: Add gdk_drag_action_is_unique()

This will be necessary once we remove the ASK action.

docs/reference/gdk/gdk4-sections.txt

@@ -798,6 +798,9 @@ gdk_drag_context_get_dest_surface
 gdk_drag_context_get_drag_surface
 gdk_drag_context_set_hotspot
 
+<SUBSECTION>
+gdk_drag_action_is_unique
+
 <SUBSECTION Standard>
 GDK_DRAG_CONTEXT
 GDK_TYPE_DRAG_CONTEXT

gdk/gdkdnd.c

@@ -876,3 +876,22 @@ gdk_drag_context_handle_dest_event (GdkEvent *event)
   gdk_drag_context_commit_drag_status (context);
   return TRUE;;
 }
+
+/**
+ * gdk_drag_action_is_unique:
+ * @action: a #GdkDragAction
+ *
+ * Checks if @action represents a single action or if it
+ * includes multiple flags that can be selected from.
+ *
+ * When @action is 0 - ie no action was given, %TRUE
+ * is returned.
+ *
+ * Returns: %TRUE if exactly one action was given
+ **/
+GdkDragAction
+gdk_drag_action_is_unique (GdkDragAction action)
+{
+  return (action & (action - 1)) == 0;
+}
+

gdk/gdkdnd.h

@@ -97,6 +97,9 @@ GdkSurface       *gdk_drag_context_get_source_surface    (GdkDragContext *contex
 GDK_AVAILABLE_IN_ALL
 GdkSurface       *gdk_drag_context_get_dest_surface      (GdkDragContext *context);
 
+GDK_AVAILABLE_IN_ALL
+GdkDragAction     gdk_drag_action_is_unique            (GdkDragAction   action);
+
 /* Destination side */
 GDK_AVAILABLE_IN_ALL
 void             gdk_drag_status        (GdkDragContext   *context,