drop: Add a convenience api

This is a question that drag-drop handlers frequently
need to answer, so make it easy.

gdk/gdkdrop.c

@@ -1001,3 +1001,30 @@ gdk_drop_emit_drop_event (GdkDrop  *self,
   gdk_drop_do_emit_event (event, dont_queue);
 }
 
+/**
+ * gdk_drop_has_value:
+ * @drop: a #GdkDrop
+ * @type: the type to check
+ *
+ * Returns whether calling gdk_drop_read_value_async() for @type
+ * can succeed.
+ *
+ * Returns: %TRUE if the data can be deserialized to the given type
+ */ 
+gboolean
+gdk_drop_has_value (GdkDrop *drop,
+                    GType    type)
+{
+  GdkContentFormats *formats;
+  gboolean ret;
+
+  formats = gdk_content_formats_ref (gdk_drop_get_formats (drop));
+  formats = gdk_content_formats_union_deserialize_gtypes (formats);
+
+  ret = gdk_content_formats_contain_gtype (formats, type);
+
+  gdk_content_formats_unref (formats);
+
+  return ret;
+}
+

gdk/gdkdrop.h

@@ -92,6 +92,9 @@ char *                  gdk_drop_read_text_finish       (GdkDrop
                                                          GAsyncResult           *result,
                                                          GError                **error);
 
+GDK_AVAILABLE_IN_ALL
+gboolean                gdk_drop_has_value              (GdkDrop                *self,
+                                                         GType                   type);
 
 G_END_DECLS