Add custom Better Victory Screen statistics

[heinwessels]

Adds a custom statistic to the Better Victory Screen, if installed. Currently only added "Mineral sludge consumed", but it's very easy to add more. And I'm open to suggestions, but would prefer if more stats was added by the maintainers :) Was thinking about:

• Landfil placed (slow to do with count_tiles_filtered, like a few seconds, and complex to count dynamically while placing)
• Total viscous mud water pumped

[heinwessels]

Added a rought count of the amount of landfil placed. This is just an approximation, but the size of base required to finish this mod the effect of generated islands will be minimal. I made an suggestion on the forums to be able to track this value better, because having an accurate count is quite complicated currently.