0.2.2: rectangle widget, text labels, and the readout for the host

[TheDrOnos]

Three additions Ground Crew's DQE mode consumes, each a thin wrapper in the shape of the existing circle/line ones:

• FigureView.add_rectangle_widget: a draggable rectangle with an optional max_extent size cap; on_change(x, y, w, h) fires when a drag settles. Replaces a stopgap in Ground Crew that reached FigureView._plot2d.
• FigureView.add_texts: text labels at image-pixel positions, one colour or one per text, with an optional halo; the same name replaces the group in place.
• FigureView.set_readout_visible and FigureFrame's onReadout: every figure frame relays anyplotlib's hover readout (apl:readout) to the host page, so an app can hide the on-image pill and print the position and value in its own units. The listener keeps only messages from its own iframe, so a figure mounted twice serves each mount.

anyplotlib floor raised to 0.10.0 (the readout event and text halos).

Confirmed end to end in Ground Crew against this branch: its DQE e2e drives the real hover relay and the rectangle widget.