Delete singleplayer

See merge request STJr/SRB2!870

Standardize the behavior of FF_INVERTSIDES and FF_INVERTPLANES

See merge request STJr/SRB2!863

Demos refactor

See merge request STJr/SRB2!853

P_HomingAttack related fixes

See merge request STJr/SRB2!836

Fix lib_setSpriteInfo indexes being off-by-one

See merge request STJr/SRB2!854

Updates and fixes for the ZB config

See merge request STJr/SRB2!859

Delete slopes

See merge request STJr/SRB2!857

Skip empty gametypes in level select menu

See merge request STJr/SRB2!819

Remove the multi-file ZB configs. We're stick with the single-file config for ZB and doing all the UDMF stuff in UDB

# Conflicts:
#	src/g_game.c

(also update g_demo.c)

Delete Lua

See merge request STJr/SRB2!852

Create G_FreeGhosts, for the benefit of G_DeferedInitNew (assuming it actually needs to do `ghosts = NULL;` at all)

Expose more music-related functions to Lua

See merge request STJr/SRB2!817