Store custom FOF flags directly in back toptexture field at map load instead of re-reading the sidedefs data later on

-Add linedef pointer to side_t, so sidedefs are able to tell if they're a front or back sidedef during setup
-Fix a broken condition during setup of texture fields for the change music linedef

This reverts commit 2cf94ea6.

Fix logfiles crashing Windoze (resolves #102)

Closes #102

See merge request STJr/SRB2!591

Hack P_SetupLevel into little pie- actually just clean it up a bit

See merge request !587

Remove MF_NOTHINK from join-breaking referenced objects

See merge request !589

Rename P_LoadThingsOnly to P_RespawnThings to make it clearer that it doesn't actually reload the things from the file, it just respawns them.

Rename P_SetupLevel to the more accurate P_LoadLevel and separate out some of its content into subfunctions

More rollangle changes

See merge request STJr/SRB2!582

Fix edge case with R_SkinUsable

See merge request STJr/SRB2!573

I got a compiler warning from P_PlayerHitsPlayer being too big

See merge request STJr/SRB2!581

Zone Builder configuration refactor

See merge request !570

haha log files

See merge request STJr/SRB2!567