Add more actions for slope copying
This branch adds 9 new linedef types relating to slopes:
- 723 - Copy Backside Floor Slope from Line Tag
 - 724 - Copy Backside Ceiling Slope from Line Tag
 - 725 - Copy Backside Floor and Ceiling Slope from Line Tag
 - 730 - Copy Frontside Floor Slope to Backside
 - 731 - Copy Frontside Ceiling Slope to Backside
 - 732 - Copy Frontside Floor and Ceiling Slope to Backside
 - 733 - Copy Backside Floor Slope to Frontside
 - 734 - Copy Backside Ceiling Slope to Frontside
 - 735 - Copy Backside Floor and Ceiling Slope to Frontside
 
Test executable: srb2win_slopecopy.exe
Test level: slopecopytest.wad
Edited  by sphere
