Issue #280: Add Hem tool (dismine/valentina)

New issue 280: Add Hem tool https://bitbucket.org/valentinateam/valentina/issue/280/add-hem-tool

Susan Spencer:

To add a curved or complex hem, we may need to add a specific Hem tool.

On the pattern, we draw a dashed lines and/or curves at the hem depth above the bottom edge of the pattern Select ‘Hem’ tool, then select the points and curves along the bottom edge of pattern, then select the points and curves along the dashed line, then add amount for turnunder at top of hem. This tool would ‘mirror’ the dashed line on the other side of the hem to create the pattern with hem. The Hem should be created after any adjacent seam allowances have been created.