improved finding a function file for editing in gui (bug #41509) * octave-qscintilla.h: new signal context_menu_edit_signal * octave-qscintilla.cc (contextmenu_edit): only emit a new signal, handling of this menu action has been moved to file_editor_tab * file-editor-tab.h: new slot handle_context_menu_edit * file-editor-tab.cc (constructor): connect new signal from octave-qscintilla to the related new slot for handling the edit function event; (handle_context_menu_edit): new slot searching for a matching subfunction, a private function or one within the same directory when __which__ returns an empty type; old contents from contextmenu_edit has moved here as well