- correct all icon usage. use 'CANCEL' instead of 'ERROR' to indicate error correctly. and move these icon const into UTILS_icons_manager.py
- auto rename generated rail section to match the default name standard.
- give more clear signal about import and export bm file by using Operator.report() function.
- add tunnel section creation and re-organise rail creation menu.
- allow duplicated elements (Nong xxx) creation for some special, such as Fan and Extra Point.