Adds flatpack stacking

Created by: ComicIronic

You can now stack flatpacks to create flatpack stacks. Flatpacks can't be placed on stacks when opened, or opened when in a stack. Stacks can be separated contextually through clicking. You can break up one stack into multiple stacks if you click in the middle of the stack. Some flatpacks now also have a departmental icon for easier identification. Example : http://puu.sh/hAw2P/0bf3a303b2.webm