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fixes#3974
Snapping to 45 degrees during arc creation (hold ctrl during an arc within a polyline), resulted in
reduced precision, because Gui::Command::doCommand %f defaults only to six decimal positions, which is
a poor representation in radians of, for example, 90 degrees.
A work-around could have been to hardcode a higher number of decimals, as in %0.Xf. However, I do not like
such magic numbers.
The solution chosen is to use an App.Quantity object using as units degrees, which leads to no loss of precision.
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