add menu entries and shortcuts for zoom functions in the editor (bug #41516) * file-editor.cc (zoom_in): slot for zoom-in action, triggering signal for tab; (zoom_out): slot for zoom-out action, triggering signal for tab; (zoom_normal): slot for zoom-to-normal action, triggering signal for tab; (construct): new View-menu with zoom-in, zoom-out and zoom-normal entries; (add_file_editor_tab): connect zoom-signal to newly created tabs; (set_shortcuts): set shortcuts for new zoom actions; (check_actions): disable zoom action when no editor tab is open * file-editor.h: zoom-in, zoom-out and zoom-normal actions, related slots and related signals to the actual tab * file-editor-tab.cc (zoom_in): new slot for signal from file_editor; (zoom_out): new slot for signal from file_editor; (zoom_normal): new slot for signal from file_editor * file-editor-tab.h: new slots zoom_in, zoom_out, zoom_normal * octave-qscintilla.cc (eventFilter): event filter for overriding qscintilla internal shortcuts; (constructor): install event filter * octave-qscintilla.h: event filter