[FEATURE]: add option to export all lines with minimal width in dxf export

[mhugent]

This PR adds an option to the dxf export to write all lines with minimal width 0 (hairline) if enabled. DXF-Lines having zero width stay minimal in the CAD regardless of the zoom-level, so this is quite usefull for doing further CAD-editing with the exported dxf, especially if there are many features next to each other on the map.

Going to add a unit test and support for access via server and processing to the PR.

The option can be enabled/disabled in the dxf dialog:

Funded by Stadtwerke München

[nyalldawson]

Is there a clearer checkbox label we can use here? Should we just go with an explicit "export lines with zero width"?

[mhugent]

Is there a clearer checkbox label we can use here? Should we just go with an explicit "export lines with zero width"?

'export lines with zero width' sounds good

[qgis-bot]

@mhugent

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[qgis-bot]

@mhugent
A documentation ticket has been opened at qgis/QGIS-Documentation#9070
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
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[qgis-bot]

@mhugent
This pull request has been tagged as requiring documentation.

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