0.13.0 — Labeled boxes

Added

• drawing_mode="labeled_rect": a rect tool that stamps every box drawn
  with a label chip. New label: str = "" parameter -- Python supplies the
  current label, and each box drawn is stamped with whatever it was at the
  time; changing label between draws does not relabel boxes already on the
  canvas. label="" (the default) draws a box with no chip. Passing a
  non-empty label in any other drawing_mode raises ValueError.
  The chip sits above the box's top-left corner at a constant screen size
  regardless of the box's scale, and flips inside the box near the top of
  the canvas. Its colour is derived from stroke_color, with the text
  colour auto-picked for contrast; its font size reuses font_size.
  Labeled boxes are always axis-aligned (no rotation handle) and are
  excluded from the toolbar edit toggle's point editing.
• Relabeling: with the toolbar's edit toggle on, click an
  already-selected labeled box a second time to edit its label through a
  transient text field -- the same gesture that re-enters an IText or
  descends into point editing. One undo entry per relabel.
• CanvasResult.boxes: every labeled box on the canvas as
  {label, left, top, width, height} dicts, in canvas pixels, with the
  scaleX/scaleY correction already applied.
• CanvasResult.boxes_in_image_space: boxes converted into
  background_image's source pixels. None when there is no background
  image.
• CanvasResult.background_fit: the background image's natural size
  and applied scale/offset. None when there is no background image.
• boxes_to_drawing(): a module-level function building an
  initial_drawing from CanvasResult.boxes-shaped dicts -- the exact
  inverse of boxes.
• font_size now also applies to drawing_mode="labeled_rect"'s chip text,
  in addition to drawing_mode="text". No behaviour change for existing
  calls.

A drawing containing a labeled box needs this version or later to load --
on an earlier version (or any other app embedding Fabric.js directly
without this component's class registered), loading it fails entirely and
none of the canvas's objects appear, not just the labeled ones.