Place a dimension that would over-constrain the sketch as a driven one (#366)

Sketch Dimension refused a dimension the sketch couldn't take, with
"that dimension would over-constrain the sketch; add it as a driven
(reference) dimension with driven: true", which the tool gave no way to
do. It now places it as a driven dimension instead: while placing, the
preview shows it in parentheses in the driven colour, and the click adds
it at once (it has no parameter and no value to edit), with a status line
saying why. Other refusals, such as an unsupported pick, are reported as
before, since the driven dimension is refused too.

Refs #320

Co-authored-by: Vitali M. <[email protected]>

docs/book/src/guide/sketching.md

@@ -112,6 +112,10 @@ Between two points, or on a line, that don't line up with an axis, where you pla
 dimension picks what it measures: across from them, the distance between them; above or below
 them, the horizontal distance; beside them, the vertical distance.
 
+A dimension the sketch can't take, because the geometry is already fixed by other constraints
+and dimensions, is added as a driven (reference) dimension instead: it is shown in
+parentheses, measures the sketch, and has no value to edit.
+
 For **Coincident**, pick two points, a point and a curve (in either order), or two
 distinct straight lines. Two lines join their closest pair of endpoints; the first
 picked line stays in place when the existing constraints allow it. Pick endpoints