Provide functionality similar to HTML table rowspan function

The idea is about be able to define virtual "rowspan" for "Container" object in Sintel Forms Designer, like on the w3school post below

https://www.w3schools.com/html/tryit.asp?filename=tryhtml_table_rowspan

That'll help designing forms where there's less whitespace and therefore it'll provide better end-user experience in sections where there's odd number of fields to be added.