try harder to find editor tab widgets for files with multiple names * file-editor.h, file-editor.cc (file_editor::editor_tab_map_iterator, file_editor::editor_tab_map_const_iterator): New typedefs. (file_editor::find_tab_widget): New function. (file_editor::request_open_file, file_editor::check_conflict_save, file_editor::handle_delete_debugger_pointer_request): Use it.