Move the BOM import dialog's row building, grouping and per-group nest
construction out of BomImportForm into OpenNest.IO/Bom so they can be
tested on Linux:
- BomPartRow (now public) and BomImportRows.Build, a verbatim move of
the form's BuildPartRows (still via BomAnalyzer).
- BomImportGroups.Build: one grouping used by both the Groups tab and
Create Nests, which each carried their own copy. Create Nests now
creates nests in the Groups tab's order (material, then thickness).
- BomNestBuilder.Build: the nest for one group (saved defaults first,
then the group's plate size, spacing, material and thickness; one
drawing per row with the row's quantity, 1 when blank).
The form keeps the file dialogs, grids, EditNestForm windows and the
completion message. Behavior is otherwise unchanged; the two row-status
defects found while planning are fixed in the following commits.
