The Corner Radius field in the Properties and Transform panels has a
link toggle behind it (the "singleCornerRadius" preference, on by
default). When the link is on, one field sets every corner, as before.
When it is off, the panels show one field for each corner of a live
rectangle: Top Left, Top Right, Bottom Right and Bottom Left. Each field
sets its own corner through object.setLiveShape {corners, radius}.
This is the part of #1594 that does not change the corner widget drag
or its modifier keys, split out for review.
Tests: a Properties panel test (the link on shows one field; off shows
the four labelled fields with each corner's radius, in line and in
order). The new labels are translated in all 15 catalogs.
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