SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
This adds an option "Lenient" that makes the reader conform to what
appears to be the norm in spreadsheet-land:
- Treat missing values as empty ones
- Update previously read rows if another row with more columns are seen
afterwards
This broke in 6a6f19a72f, which replaced
the representation of columns with numbers.
As a result, the save logic would store cells as
"\x<column_index><row_number>", which is obviously wrong.
Fixes#5905.
Also simplifies the control flow in `import_worksheet` a bit.
