Improve Piece path validation. Check uniqueness.

In most strict case Valentina doesn't allow double points in a piece path. By double point i mean the first and the last or pair with the same coordinates. This prevents drawing weird path and seam allowance. But, in some cases this rule is too much strict because our goal is to build flexible patterns and they should be able to change shape freely.

For example in case of dart it can be closed of open. In case of closed two points will have the same coordinates and we don't want replace them by one point.

I propose add special mark - check uniqueness. By default the check enabled, but if user think Valentina should ignore check for uniqueness for this point he can disable it. Label in this case will be shown inside of square brackets.

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