Suggestion: UI to configure Reserved Barrels/Bins

[quietust]

Back in versions 0.47.05 and earlier (going way back to one of the early 2D versions), the Stockpile menu contained controls for setting aside a number of Barrels or Bins to reserve them for workshop jobs (e.g. brewing alcohol or extracting Gnomeblight). Toady apparently neglected to expose these in the new UI, but the underlying variables are still present (in plotinfo.stockpile) so it should just be a matter of exposing them in some manner.