Add support for HTML bold/italic/font size and font family to labelling

[nyalldawson]

This PR adds support for additional HTML formatting tags within labeling (and everywhere text renderer is used). Specifically:

• font family
• font size
• bold
• italic

Sponsored by the Swiss QGIS user group

Fixes #40532
Fixes #32098
Fixes #18727
Fixes #31501

[nirvn]

@nyalldawson , did some testing, it's really nice, good job.

I did run into a crash (having to do with curved labels I think):

#6  0x000055555556567b in myMessageOutput(QtMsgType, QMessageLogContext const&, QString const&)
    (type=QtFatalMsg, msg=...) at /home/webmaster/dev/cpp/QGIS/src/app/main.cpp:455
#7  0x00007ffff3a2e530 in  () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#8  0x00007ffff3a2e649 in  () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#9  0x00007ffff39fab8a in QMessageLogger::fatal(char const*, ...) const ()
#10 0x00007ffff39f9ff0 in qt_assert_x(char const*, char const*, char const*, int) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#11 0x00007ffff142a73d in QList<double>::last() const (this=0x7fffcbffb378) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qlist.h:365
#12 0x00007ffff142a134 in QList<double>::constLast() const (this=0x7fffcbffb378) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qlist.h:366
#13 0x00007ffff1429a41 in QgsTextDocumentMetrics::calculateMetrics(QgsTextDocument const&, QgsTextFormat const&, QgsRenderContext const&, double)
    (document=..., format=..., context=..., scaleFactor=1) at /home/webmaster/dev/cpp/QGIS/src/core/textrenderer/qgstextdocumentmetrics.cpp:153
#14 0x00007ffff1e0150b in QgsPalLayerSettings::calculateLabelSize(QFontMetricsF const*, QString const&, double&, double&, QgsFeature const*, QgsRenderContext*, double*, double*, QgsTextDocument*, QgsTextDocumentMetrics*)
     (this=0x55556062e520, fm=0x7fffc07238c0, text=..., labelX=@0x7fffcbffb4e8: 2.5599981057214506e-172, labelY=@0x7fffcbffb4f0: 9.8813129168249309e-324, f=0x7fffcbffbaa0, context=0x55556062b5d0, rotatedLabelX=0x7fffcbffb4f8, rotatedLabelY=0x7fffcbffb500, document=0x7fffcbffb508, documentMetrics=0x7fffcbffb890) at /home/webmaster/dev/cpp/QGIS/src/core/labeling/qgspallabeling.cpp:1630
#15 0x00007ffff1e03f30 in QgsPalLayerSettings::registerFeatureWithDetails(QgsFeature const&, QgsRenderContext&, QgsGeometry, QgsSymbol const*)
    (this=0x55556062e520, f=..., context=..., obstacleGeometry=..., symbol=0x55556062c2d0)
    at /home/webmaster/dev/cpp/QGIS/src/core/labeling/qgspallabeling.cpp:2025
#16 0x00007ffff1e2e9f4 in QgsVectorLayerLabelProvider::registerFeature(QgsFeature const&, QgsRenderContext&, QgsGeometry const&, QgsSymbol const*)
    (this=0x55556062e4c0, feature=..., context=..., obstacleGeometry=..., symbol=0x55556062c2d0)
    at /home/webmaster/dev/cpp/QGIS/src/core/labeling/qgsvectorlayerlabelprovider.cpp:199
#17 0x00007ffff1fe3db2 in QgsVectorLayerRenderer::drawRenderer(QgsFeatureRenderer*, QgsFeatureIterator&)
     (this=0x55556062bf10, renderer=0x55556062d7a0, fit=...) at /home/webmaster/dev/cpp/QGIS/src/core/vector/qgsvectorlayerrenderer.cpp:511
#18 0x00007ffff1fe3039 in QgsVectorLayerRenderer::renderInternal(QgsFeatureRenderer*) (this=0x55556062bf10, renderer=0x55556062d7a0)
    at /home/webmaster/dev/cpp/QGIS/src/core/vector/qgsvectorlayerrenderer.cpp:409
#19 0x00007ffff1fe1a6c in QgsVectorLayerRenderer::render() (this=0x55556062bf10)
    at /home/webmaster/dev/cpp/QGIS/src/core/vector/qgsvectorlayerrenderer.cpp:225

I haven't worked out a reduced test project to replicate crash, but on a large project, I can replicate crash as soon as curved labels are on the canvas. Let me know if you need something or the above stacktrace is enough.

One thing I've noticed is the absence of support for font-size in %. Is that a Qt limitation or is there a way to support that? I'd be nice to have it as you could still control the text size via UI and have parts be e.g. font-size:80% of the text size that's set via UI.

In any case, super cool stuff.

[nirvn]

The code change is, well, dense! :) I couldn't find anything wrong but it's not the easiest part of the src tree to navigate. I however did a fair amount of testing locally, and couldn't spot any further regressions. Good job. Just a few Qs.

[github-actions]

@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]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#7762
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!