editor: add list of recently used files to the file menu * file_editor_tab.cc (set_file_name): emit signal mru_add_file () for a new file * file_editor_tab.h: new signal mru_add_file () * file_editor.cc (request_mru_open_file): new handler for opening a file from the mru-list, (handle_mru_add_file): slot for signal emitted from file_editor_tab; adds the new file to the mru list, (mru_menu_update): private function for updating the mru-list, (mru_menu_update): private function for updating the mru-list, (construct): implement mru-list menu and the related actions, (add_file_editor_tab): connect signal mru_add_file () * file-editor.h: definition of new slots, methods, string list for mru-list, and array for the related actions