OK button in Piece Dialog erroneously disabled

@philip reported to me that a bug exists regarding the OK button in the Pattern Piece dialog. When you attempt to edit anything in the dialog when creating a piece the OK button is disable when it shouldn’t be. The result is that you can no longer save the piece, and have to start over. This is a bug that appeared after fixing the bug validating the before & after seam allowances when editing a piece.

I have created a PR with the fix. The dialog no longer tries to validate the before & after seam allowances while creating a piece… only when in edit mode when the formulas are likely to be valid or can be edited.