nemo-view.c: Clear extension menu items during real_update_menus.

Extension menu items take a reference to the file selection, and
this reference isn't cleared until the next time the extension
menu items are fetched (via  GClosureNotify), which, as of f5124bc
is only done during a right-click.

Instead, clear the extension actions when real_update_menus runs,
which includes after a selection change, and during view location
changes.

Fixes linuxmint/mint20.2-beta#20.

src/nemo-view.c

@@ -5648,6 +5648,8 @@ add_extension_action_for_files (NemoView *view,
 	gtk_action_set_sensitive (action, sensitive);
 	g_object_set (action, "is-important", priority, NULL);
 
+    // FIXME: Don't add the selection to the action data.
+    // Just grab the selection in the activation callback.
 	data = g_new0 (ExtensionActionCallbackData, 1);
 	data->item = g_object_ref (item);
 	data->view = view;
@@ -9604,6 +9606,7 @@ update_configurable_context_menu_items (NemoView *view)
 static void
 real_update_menus (NemoView *view)
 {
+    GtkUIManager *ui_manager;
 	GList *selection, *l;
 	gint selection_count;
 	const char *tip, *label;
@@ -10033,6 +10036,11 @@ real_update_menus (NemoView *view)
                                         !selection_contains_recent && !selection_contains_trash);
     }
 
+    ui_manager = nemo_window_get_ui_manager (view->details->window);
+    nemo_ui_unmerge_ui (ui_manager,
+                        &view->details->extensions_menu_merge_id,
+                        &view->details->extensions_menu_action_group);
+
     update_configurable_context_menu_items (view);
 
     nemo_file_list_free (selection);