Sound Filter options (sndspeed but more UX friendly)

[drHyperion451]

Adds a "Sound filter" option under "game" section in options. It switches on-off the low-pass filter which is applied when the game is resampling a 11.125 kHz audio to 44.1 kHz (so it rids off the "crunch-ness" effect that creates the resampling.

This is a continuation of #314 which was mistakenly closed because I made a force pull request. In that thread we discuss how to solve this problem.

Also this changes the sndspeed option to be snd_filter for some reasons:

1. There's already a snd_filterquality variable, that does exactly what it says, so it fits well with snd_filter
2. sndspeed was either way a switch between 11125 and other values. It's pointless to have an exact numerical value and leaves confussion. A boolean option should be more intuitive.
3. Bad naming in my viewpoint. What is sound "speed"?
4. Theres already snd_mixspeed that actually does change the sample rate in more intervals.

snd_filter is now a boolean value instead of a boolean value hidden under an int value. Also this cvar is persistent in the user options (unlike sndspeed)

Still I should change the on/off switch to a more suitable "classic vs remastered" or "retro vs off" to give more hints to the user (like how it's done in the models option under the graphics section). When I do change it I will mark this PR as ready.

I'm not an expert or a professional, any feedback will be appreciated. It's a small change but pretty necesary (#169)

[drHyperion451]

Read @L-P comment too late. Closing this PR because it's unnecesary now. (#314 (comment))