Mark several cards and move them together

Cards on the canvas can now be marked with Shift-click or by drawing a lasso with Shift held, and dragging one marked card moves all of them. Grouping is still a click away: the Group button over the marked cards, or a right-click on one of them, opens the colour ring. A click on empty space or Escape lets go of the marks. Short hints show first-time users how it works until they have used it once.