improve opening files from gui when built without qscintilla (bug #39179) * external-editor-interface.cc/h: new class with an interface for an external editor, which is used by preference or if qscintilla is not available; (external_editor_interface): ctor, connect a signal for opening the settings; (external_editor): check for an external editor being set in the settings; (call_custom_editor): call the custom editor; (request_open_file, request_open_file, handle_edit_file_request): slots for the various ways for opening a file * file-editor-interface.h: moved request_new_function and removed request_open_file into the main window * file-editor.cc: include external-editor-interface.h; (file_editor): file encoding tracked in main window; (request_new_function): moved into main window; (request_open_file (void)): moved to main window; (request_open_files): moved to main window; (handle_combo_enc_current_index): moved to main window; (restore_create_file_setting): moved into main window; (request_new_script): moved into main window; (restore_create_file_setting): moved into main window; (call_custom_editor): use external_editor_interface; (handle_edit_mfile_request): moved into main window; (construct): moved some signal connection into the main window; (add_file_editor_tab): update signal connection to moved slots; * file-editor.h: moved request_new_function, handle_edit_mfile_request, request_open_file (void), request_open_files, handle_combo_enc_current_index, and class variable _file_encodinginto the main window; * main-window.cc (main_window): initialize _file_encoding to empty string, set external editor interface and store active editor for connecting signals; (~main_window): delete external_editor_interface; (edit_mfile): directly use slot handle_edit_mfile_request moved here; (handle_edit_mfile_request): moved from editor to here; (request_new_function): moved from editor to here; (request_open_file): moved from file editor to here; (set_file_encoding): moved from file editor to here, old name was handle_combo_enc_current_index; (request_open_files): moved from file editor to here; (restore_create_file_setting): moved from editor to here; (construct_octave_qt_link): connect signals to slots in editor or external editor interface; (construct_file_menu): connect signals to slots in editor or external editor interface, slot for opening files now in main window (construct_new_menu): connect signals to slots in editor, external editor interface, or to slots moved here; * main-window.h: update signals according to moved slots, namely request_new_function, request_new_script, handle_edit_mfile_request, restore_create_file_setting, moved request_open_file, set_file_encoding, request_open_files, and class variable _file_encoding to here * module.mk: added new files external-editor-interface.cc/h