Heretic umapinfo support

[Pedro-Beirao]

Issue #530

This is an attempt to implement umapinfo support in the current intermission/finale codebase. I will want to rewrite/refactor some of these files to better organize the specific intermission/finale code for doom, heretic and hexen, and the implementation of umapinfo. Currently, heretic and hexen are just "patched in" with the old doom code, making it hard to work with

Since there is no spec for umapinfo in heretic, I went with a mix of: do like vanilla heretic and do like doom's umapinfo

levelname, label, author, next, nextsecret, skytexture, music, endgame, endpic, nointermission, episode - simple implementation

partime - heretic does not have partimes. now it displays it like doom

exitpic, enterpic - in the first 3 eps, heretic has 3 intermission screens: 'status', 'finished' and 'entering'. in the other eps, it only has 1, the 'status' screen. here, i followed this behaiviour. but should it instead be like doom and only have a 'status' and a 'entering' screens on all eps?

endbunny, endcast - not implemented since they do not translate to heretic

intertext, intertextsecret, interbackdrop, intermusic - implemented, but you are not able to skip it by pressing any key, just like vanilla heretic

bossaction - not implemented, we need a spec detailing this

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So, lets figure out what should be the correct behaviours for umapinfo in heretic, and we can either go for a "patch in" solution like this, or we could wait till get arround to refactor the intermission/finale codebase (from wi_stuff.c downwards) for a cleaner implementation.

Test wad: heretic-umapinfo.wad.zip