Add volume setting to ini

[dagit]

Description

I found that I kept resetting the volume level every time I loaded up the game. So I added a config option to the ini file.

Will this Pull Request break anything?

It shouldn't but I only tested it on linux.

Suggested Testing Steps

Change the volume setting in the ini file. The range is probably platform dependent, but SDL seems to go from 0 to 128 and it looks like windows probably ranges from 0 to 100. So the ranges aren't that different hopefully.

[snesrev]

SDL_MIX_MAXVOLUME goes from 0-128 not 0-100, it needs to be 0-100 in the config file and that should be translated to 128 for SDL. Also is there a way to use less #ifdef

[dagit]

I refactored it so that the current mixer volume is internally always in the range 0 to 100, but when updating the SDL mixer it scales that value into the range 0 to 128.

I don't really see a way to use less #ifdef here. There's a bit of fundamental complexity due to using two different mixer systems. I've tried to hide that complexity behind the getter and setter functions so that the rest of the code can be written as if there is just one API. I believe that's a pretty decent trade-off and if you want me to do it a different way you'll have to explain what you have in mind.