BACKPORT: removal of music slots

Relevant commits cherry-picked. Basically everything except the internal music track name switches.

Added 55 new commits:

• b3b5521e - Merge branch 'master' into next
• e43f21cd - Fix dead MF_PAIN objects hurting the player
• 8cf74f50 - Merge branch 'mf_pain-fix' into 'next'
• 31deecc5 - Colour Changing MD2s
• 6b8c438e - Change a few colours.
• ddb5652a - Tabbing
• 6000b5c9 - Merge branch 'master' into next
• 5d1c8d29 - My cherry picking somehow lead to these functions being doubled ...whoops
• 7c5adacc - Fixed chain launching
• 1e15ed2f - Merge branch 'i-hate-stupid-speedrun-tricks' into 'next'
• 5108bb97 - Merge branch 'master' of http://git.magicalgirl.moe/STJr/SRB2 into pub_next
• 8b7e0347 - Merge branch 'master' of http://git.magicalgirl.moe/STJr/SRB2 into pub_next
• cb954284 - Merge remote-tracking branch 'public/next' into next
• 66175d87 - removed unused macros from doomdef.h
• db18ad6b - Merge branch 'master' of http://git.magicalgirl.moe/STJr/SRB2.git into monster-misc-2
• 8cc6261c - Merge branch 'monster-misc-2' into 'master'
• 662d4051 - travis: let start building with travis
• 69937b41 - travis: simple build config
• 0003cea5 - travis-ci: we need sudo
• 14440675 - travis: use the correct packages
• 522d2049 - travis: add cmake 3.x package
• 45b181f8 - travis: switch to trusty for linux os
• 0bd16a5a - travis: drop osx
• 5e5c73fe - travis: add SRB2 2.1.14 files
• a743f155 - travis: addd libgme to list
• b6cc13fd - travis: drop test
• 152c106c - travis: restore language
• 59d111b1 - travis: restore missing config?
• e913ca0c - travis: use wget and create cache folder
• ae75e3ee - travis: mkdir , not make and cache the cccahe
• a979e375 - travis: no space on 7z command
• 4d5f1633 - travis: just run make
• 864baeda - cmake: fixed up to handle Clang and AppleClang
• f4886657 - clang: fixup a few clang warnings
• 68054a49 - clang: cleanup
• 07fc74ea - clang: fixup a few clang warnings
• 7c1f1d9c - travis: compile with warnings as errors, skip absolute-value warnings
• 6d26a60c - travis: cache 7z SFX
• e9048aec - travis: premake the cache folder
• 3ba827f3 - Merge branch 'master' into travis-ci
• 914c6c9d - Whitespace cleanup.
• 985b9a11 - Merge branch 'travis-ci' into 'master'
• cc3d3a67 - Colour Changing MD2s
• d050a60f - Change a few colours.
• 6a67609b - Merge branch 'master' into md2_colourchange
• 7d914913 - Tabbing
• 700c9c2e - Merge branch 'md2_colourchange' into 'master'
• b258b9b5 - remove cpuaffinity code from SDL
• 6dda71be - I guess this is becoming the "try to make SDL_mixer work" branch
• aa4414d7 - Merge branch 'sdlmix-master' into 'master'
• 8a369d96 - whitespace clean
• 9b55244d - travis: use SRB2-v2114-assets.7z
• 41f8b25c - travis: add verbose and server-response to wget
• 6655e924 - Merge branch 'master' into next
• 2b12e5ff - Merge branch 'next' into backport_unslot-music

Added 1 new commit:

• f5ba192f - remove extraneous va() calls

Added 1 new commit:

• 7349cbdb - Backwards compatibility.

Added 1 new commit:

• 61a0d1bc - don't use lstring

mentioned in commit a8bca89d

Status changed to merged