automatic completion list as user preference (bug #41469) * file-editor-tab.cc (constructor): remove unnecessary call for completion list; (show_auto_completion): new slot showing autocompletion list at cursor; (notice_settings): auto completion properties are set even if automatic showing of the list is disabled, only treshold is updated * file-editor-tab.h: new public slot show_completion list * file-editor.cc (request_completion): new slot for completion-list-action triggering a signal to the actual editor tab; (construct): new action in edit menu for showing completion list, the signal is connected to request_completion; (add_file_editor_tab): connect signal for showing the completion list to the new editor tab's slot show_auto_completion; (set_shortcuts): set shortcut for new action; (check_actions): disable new action when no tab is present * file-editor.h: new action, new slot, new signal for tab * settings-dialog.ui: update the settings dialog accordingly