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

(types 730 to 735 in action, from left to right)