More thinker refactoring
Move a bunch of linedef special settings from the thinker functions into P_SpawnSpecials, to keep the differences with udmf-next localized. Based on !899 (merged), so that one should be merged first.
Move a bunch of linedef special settings from the thinker functions into P_SpawnSpecials, to keep the differences with udmf-next localized. Based on !899 (merged), so that one should be merged first.