Draft: Add "legacy importer only" to DXF option

[Roy-043]

Fixes #13598.

[furgo16]

@Roy-043 thanks! I think there are stilll more capabilities in the preferences dialog that are legacy-importer-only. I wonder if there is a more concise way to mark them without appending the ("legacy importer only") string to them.

I did a side by side comparison table here: #13599 (comment)

Here's a mockup of what I had in mind:

Conceivably, a better implementation would be to disable the relevant options when the Use legacy python exporter checkbox is ticked. However, as a first iteration, just marking those options would already go a long way preventing user confusion and spending time in trial-and-error importer passes. Those can take a quite a long time depending on the imported file's complexity, and the options chosen.

[Roy-043]

Thanks for the detailed analysis. It is indeed better to do this properly. I'll convert this PR to Draft for now. If I remember correctly there may even be a setting that affects both import and export...

[furgo16]

Thank you. I just finished the analysis and updated the original comment. The comparison table is now up-to-date to the best of my knowledge.

[furgo16]

Related: #13751

[Roy-043]

Result of the updated PR:

[furgo16]

This is excellent, thanks so much. This removes the previous uncertainty, it will make life easier and it will prevent unnecessary trial and error trying combinations of unimplemented options. Some other comments:

1. Is it worth considering sensible default settings in this PR, or rather discuss it in a separate issue? If so, happy to file it. Otherwise, in addition to the options already ticked in the screenshot, I'd suggest to tick the following ones as well:
   • Show this dialog when importing and exporting => once there is one single importer with feature parity with the old one, and it is reasonably stable, this setting could be unticked as default for a more seamless import/open operation. But I think for now, it's worth letting the user know what and how will be imported, and showing them how to control that.
   • texts and dimensions => I could imagine that given the low input in importing and processing them, and that they seem to be reasonably well supported, most folks would like to keep texts and dimensions from the original DXF file.
   • Use colors from the DXF file => same as above
2. I'm not familiar enough with the exporter part to be able to comment on that section.
3. I know it's not part of this PR, but since it's a safe one-liner, and it's less overhead to change it here than on a separate PR: Use Layers => Use layers. Change to sentence case to be consistent with the rest of the copy on the dialog. Particularly "Group layers into blocks"

[furgo16]

Does this PR need further testing or review before getting merged? Thanks.

[Roy-043]

The Show this dialog when importing and exporting option is now checked by default. Changing the other preferences does not necessarily make sense for other workflows where a DXF is just used as a vector format, for example to import the contour of a product.

[furgo16]

Makes sense, thanks!