XLIFF format customization

[nijel]

Describe the problem

Once file formats are customizable (see #1897), the following should be available for XLIFF:

• Closing tags behavior (currently in XML add-on)
• Whitespace handling: standard (follow xml:space) / always preserve / always normalize
• Tags handling (currently handled by two formats)

Describe the solution you'd like

Additional form should be available when choosing file format.

Describe alternatives you've considered

Current situation kind of works (choosing format variants, installing add-ons, ...), but does not provide configuration where people expect it.

Screenshots

No response

Additional context

Depends on #1897

[github-actions]

This issue has been added to the backlog. It is not scheduled on the Weblate roadmap, but it eventually might be implemented.

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