Add flag to line slopes for even more slope copying

The sort-of sequel to !1406 (merged), this branch adds some additional functionality to line slopes: when the Transfer Line flag is checked, the slope(s) will be copied to the sector on the other side of the linedef.

For example, checking the flag on linedef action 700, Slope Frontside Floor, will copy the resulting slope to the backside sector's floor as well. This also works for line slopes that affect both the front- and backside sector, like action 713 (Slope Backside Floor and Frontside Ceiling), in which case the flag will copy the resulting slopes to the frontside floor and the backside ceiling.

This could theoretically also be implemented for vertex slopes; however, I was unable to get the copying to work on them without unwanted side effects, and it seems considerably less useful for them anyway.

Test map: slopeflagtest.wad