Alternate workflow:
 - keep graphical information (symbols, connections) on graphical sheet(s)
 - but keep user added attributes in a separate, text based file

This way all attributes (of potentially multiple sheets) are collected in one
place, which, in case of component attributes, may be easier to maintain than
attributes scattered on graphical sheets.

The actual implementation in sch-rnd is non-graphical sheets, that can add
component (and network) attributes easily. The tactics is to not fill in
attributes of symbols on the graphical sheet at all, and have all user
attributes (devmap, footprint, value) in the non-graphical sheet. In this
setup the non-graphical sheet can be regarded as a "parts configuration".

Typically there would be only one non-graphical sheet per project, listing
all components, but it is possible to have multiple non-graphical sheets,
for example components grouped into separate files by their purpose or type.

