Pending Keep path data in “Use HTML invoices” after disabling feature

[Aivaras]

Currently, if one unselects “Use HTML invoices,” the entered path to wkhtmltopdf is lost. If one then re-selects the feature, one also has to re-enter the path. This unnecessarily slows down work, especially during testing. It would be more convenient if the path would be kept (greyed out) after disabling “Use HTML invoices.”