improve saving a variable in the variable editor (bug #59570) * gui-preferences-ve.h: define lists with possible save formats and with a relation between save format and suggested file extension * variable-editor.cc: include builtin-defun-decls.h and ovl.h (variable_editor_stack::save): format string as new argument, test save_default_options if format is empty, moved the saving into do_save; up in several functions/slots (variable_editor_stack::do_save): select suitable file extension for desired save format, perform the saving with Fsave; (variable_editor::edit_variable): remove connection of obsolete signal variable_editor_stack::command_signal, connect interpreter and signals mapper for saving instead; (construct_tool_bar): add button for saving with pull down menu for different formats, creating a signal mapper for the menu actions * variable-editor.h: inlcude qt-interpreter-events.h, (variable_editor_stack): remove command_signal, add slot do_save, add interpreter_event signals, slot save with format as new argument; (variable_editor): new signal mapper *m_save_mapper