Greatly reduced CollapsibleStringGroup's memory usage by making the
expanded and collapsed views share an image. Adjusted PlayerTab to only
drag with left clicks and began work on making a context menu appear on
right clicks.
Removed an unnecessary import in StatMap. Adjusted World to keep players
in a TreeMap rather than an ArrayList.
Moved CollapsibleStringGroup to Haferlib. Adjusted Player.QuantityItem
to have a useful toString. Adjusted Stat to not merge duplicate strings.