Update FileDialog button activity when file_mode is changed.

(cherry picked from commit cfac7c9a9aa45be2bf6de6097cca6c91b9d0fbf3)

scene/gui/file_dialog.cpp

@@ -833,6 +833,8 @@ void FileDialog::set_file_mode(FileMode p_mode) {
 	} else {
 		tree->set_select_mode(Tree::SELECT_SINGLE);
 	}
+
+	get_ok_button()->set_disabled(_is_open_should_be_disabled());
 }
 
 FileDialog::FileMode FileDialog::get_file_mode() const {