When there are many Draft objects in the tree view
the icon can be used to tell different objects apart quickly.
One icon is used for objects that are "straight lines",
whose Proxy is of type `Line`, `Wire` or `Polyline`; another icon
for "regular geometrical" shapes of type `Rectangle` or `Polygon`;
and another icon for "curved" objects of type `Circle`,
`Ellipse`, `BSpline`, `BezCurve`, or `Fillet`.
Other objects use the older default icon.
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