Change taxonomy generation to only change modified lines.

[alexgarel]

Currently we are regenerating the whole taxonomy when we export it.

But this proved to be difficult to use. Taxonomies have to be perfect on Product Opener side to be easy to review and merge.

We need to switch strategy:

• keep whole taxonomy file content at import time
• register where each entry start / end (if it's not yet the case, but I think it's already done)
• keep track of modified entries
• at export time take original file and only replace the lines the entry was occupying with new lines corresponding to modified entry

[alexgarel]

Done in #554