0.17.7: Added a new building - Alloy Mixer.

Alloy Mixer gives the option to mix molten metals instead of ingots.
Alloy mixer is using placeholder images that are modified Induction Smelter images. I'm no artist.
Every alloy has it's own item-subgroup instead of having one alloy subgroup.

Formatted code in tabs instead of spaces.
.pdn files are located in placeholders folder