Update version to v0.40.19
stockpiles: refactor code into separate files

The file was approaching 3000 lines and contained multiple classes,
making it a real PITA to navigate through. upcoming features would only
add more LOC, so splitting was necessary.
stockpiles: implement GUI controls
stockpiles: integrate gui plugin into dfhack.init-example
update NEWS and README with new stockpiles UI info
stockpiles: fix usage help