Music options cleanup + bugfixes

This started as a simple streamlining of the Music Options menu, but grew into a little more than that.

• Digital & MIDI music handling has been consolidated as much as possible, with the "Music Preference" option becoming the main way to switch between the two
• Fixed post-boss music being reset to boss music, when dying in a boss stage without reloading the map (Fixes #76)
• Fixed LoopTitle not being applied in certain situations (Fixes #273)
• Exposed MUSIC_ flags to Lua (Fixes #980)
• Removed "Let levels force reset music" option because why was this even a thing that could be overridden
• Removed leftover LENGTHMS code & associated devmode warnings