UDMF: Scrollers and pushers

Based on !1557 (merged). Merge that one first.

Adapt the scroller and pusher linedef types to UDMF: 435, 510-535 and 541-546. I left out the push/pull point effect (linedef type 547) for now, because the plan is to make the linedef type obsolete and control it all via the thing instead. I did take the opportunity to move it to its own thinker though.

srb2win-pushers.exe