Allow relating/substituting parts within a project

[kilrah]

Example: Project X has part A in the BOM, Part A is out of stock, but Part B can be substituted for it for this project only (i.e. the existing "related parts" can't be used as that'd be global).

So basically within a BOM line have a selection for one or multiple possible replacements.

[Dane2007]

This would be extremely useful for my workflow with Part-DB.