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This reverts commit b3dc879e8b108c26c929bfbe551bcdf77f140e94.

This change breaks plasmashell startup and possible other apps as well.
Reverting so the root cause can be investigated without time pressure.

calling deleteGroup only deletes all entries but the group does still
exists in listGroups

this is somewhat irritating since calling exists on that group will
return false

with this patch the group does no longer exists in listGroup

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QStringView has some bits of API that were only added in 5.15.2, whereas KF
requires 5.15.0.

This reverts commit 1780fb2a237af80ddc1f9cfb70cb892b53b91990.

The symbol export/visibility attribute is not standardized so far and
needs to be set by language extension attribute declaration at least
with clang & MSVC, who both support that when it appears before the
standard attribute declaration.

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Some methods in ConfigLoaderHandler always returned true, change them to return
void instead. Also port them to take a QStringView instead of QStringRef, this
doesn't require a lot of changes because a QStringView can be constructed from
a QStringRef.

QXmlStreamAttribute methods like value() and name() return QStringRef in Qt5
and QStringView in Qt6, "fix" the issue by using auto keyword, which works
in both cases.

QStringView::toInt() isn't efficient in Qt5 so make the build conditional.

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This matches the method actually does, and makes it clearer that you don't
need to do config->sync() followed by config()->reparseConfiguration(), the
latter should suffice.