Add option 'Use only selected features' to DXF export

[gacarrillor]

Enhance DXF export by adding a handy option to filter selected features for each layer. It will be false by default.

This adds to the already existing Extent filter, so that users can apply one of these two options, or even both at the same time.

The new option has also been exposed in the corresponding Processing algorithm.

Screenshots:

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Funded by the QGIS user group Switzerland.

[qgis-bot]

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

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

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[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit bb8d095)

[nyalldawson]

This could be added to QgsDxfExport::Flag instead, and avoid the need for the new getter/setter

[gacarrillor]

Done. Much better, thanks for the suggestion.

[qgis-bot]

@gacarrillor

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