BUG: 441537
FIXED-IN: 5.86

Now that KF requires Qt 5.15.2; this basically reverts commit 2e8742e64fc0
with some trivial changes.

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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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A window that is maximized does not really have a position on screen
other then the fact that it's maximized, and writing out position data
in this case anyway will only serve to mis-position the window once
da-maximized, should KWin ever stop remembering de-maximized window
positions internally.

CCBUG: 434116

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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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All named authors from copyright statements confirmed this change
(confirmations at sysadmins' longterm archive). From the git history
check, relicensecheck tells:

- lgplv2+: mdawson pandom79@gmail.com
  mdawson (   6 LOC): 0005ba7
pandom79@gmail.com (   4 LOC): eeb2bbe

Whereas pandom79 does not posses a KDE contributor account but also
confirmed via private mail (confirmation mail also in sysadmins'
archive).

Change 0005ba7 is a syntactical replacement of QT_TRANSLATE_NOOP with
QT_TRANSLATE_NOOP3 and thus can be assumed to not by copyrightable.