Support CSS margins for block elements in HTML labels

[nyalldawson]

Adds support for CSS margin properties for block elements in
the text renderer. Specifically:

• This is available for block type elements only, eg DIV, P, H1-H6.
• Margins are NOT available for spans or other non-block content (due to Qt limitations)
• Margins can be specified in pt units only
• Margins can be specified either via "margin-left: 5pt; margin-right: 10pt" etc, or "margin: 5pt 0pt 10pt 0pt"
• Negative margins are supported for the bottom margin ONLY (due to Qt limitations)

Sponsored by City of Freiburg im Breisgau

[qgis-bot]

@nyalldawson

This pull request has been tagged for the changelog.

• The description will be harvested so please provide a "nearly-ready" text for the final changelog
• If possible, add a nice illustration of the feature. Only the first one in the description will be harvested (GIF accepted as well)
• If you can, it's better to give credits to your sponsor, see below for different formats.

You can edit the description.

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Thank you!

[qgis-bot]

@nyalldawson
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.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
Also, any commit having [needs-doc] or [Needs Documentation] in will see its message pushed to the issue, so please be as verbose as you can.

Thank you!

[github-actions]

🪟 Windows builds

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Debug symbols for this build are available here.
(Built from commit 9250db8)

🪟 Windows Qt6 builds

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(Built from commit 9250db8)

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#9376
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!