Let a lone head or tail point be dragged in line mode instead of only extended

[mattdawkins]

Stacked on #1932 (its commits are included here; this shrinks to the last commit once #1932 merges).

• A detection with only a head or only a tail (e.g. a computed keypoint) could only be extended into a line in head/tail mode; the existing point could not be moved. It now appears as a draggable handle while the line tool waits for the missing point, and clicking elsewhere still places that point.
• Reuses Keep a detection's box corners editable while editing its line #1932's companion edit layer: when the line tool is active and the track has exactly one of head/tail and no line, the companion becomes a Point editor on that key instead of the box-corner editor. Dragging commits through the head/tail recipe as a single-point edit (bounds enclose the new position), so stereo auto-compute and length updates react as they do to a placed point.
• Two GeoJS interactions handled: a mousedown on the point's handle is not a vertex of the line being drawn, and leaving the handle gives the line tool back the click-to-place actions GeoJS strips on handle hover.
• Real-GeoJS spec drags a lone head with the line tool in creation mode: the point moves, no line is started, and creation actions are restored.

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