(re-) add slime chunks option

The slime chunks option was apparently removed due to the code being "outdated". After some testing, it turns out this code still works !

This pull request adds a "Show Slime Chunks" in the View menu, and changes the slime chunks rendering a bit ("full chunks" rendered at y <= 40, green tint enhanced to be more visible).

Please note that even if I changed the code for the OSX and Win versions too, I only tested it with the Linux version.