Add generalized slopes in UDMF

This branch implements equation-based generalized slopes, which are usable with UDB's versatile visual slope tools.

When parsing a sector's fields, it will now consider the floor and ceiling plane constants. If the four of them are read for either or both planes, it will proceed to create a slope based on them; otherwise if the constants are incomplete they will be discarded.

Slope initialization now occurs earlier to allow that since they're created on mapdata parsing time, rather than being loaded via specials.