Fix GCC warning by specializing PdmVariantEqualImpl for QString

[magnesj]

Compiling on default system on Ubuntu 26.04(gcc15 and Qt6.10.2) gives a warning multiple times. This is a known false positive on GCC related to deep inlining of qvariant_cast<QString>

Fixed by specializing PdmVariantEqualImpl for QString

Building CXX object ApplicationLibCode/CMakeFiles/ApplicationLibCode.dir/ProjectDataModel/Summary/RimSummaryTable.cpp.o
In file included from /usr/include/x86_64-linux-gnu/qt6/QtCore/qhashfunctions.h:9,
                 from /usr/include/x86_64-linux-gnu/qt6/QtCore/qlist.h:11,
                 from /usr/include/x86_64-linux-gnu/qt6/QtCore/QList:1,
                 from /home/builder/gitroot/ResInsight/Fwk/AppFwk/cafProjectDataModel/cafPdmUiCore/cafFontTools.h:38,
                 from /home/builder/gitroot/ResInsight/ApplicationLibCode/ProjectDataModel/Appearance/RimFontSizeField.h:21,
                 from /home/builder/gitroot/ResInsight/ApplicationLibCode/ProjectDataModel/RimPlotWindow.h:20,
                 from /home/builder/gitroot/ResInsight/ApplicationLibCode/ProjectDataModel/Summary/RimSummaryTable.h:21,
                 from /home/builder/gitroot/ResInsight/ApplicationLibCode/ProjectDataModel/Summary/RimSummaryTable.cpp:19:
In copy constructor ‘QString::QString(const QString&)’,
    inlined from ‘T qvariant_cast(const QVariant&) [with T = QString]’ at /usr/include/x86_64-linux-gnu/qt6/QtCore/qvariant.h:900:27,
    inlined from ‘T QVariant::value() const & [with T = QString]’ at /usr/include/x86_64-linux-gnu/qt6/QtCore/qvariant.h:649:36,
    inlined from ‘static bool caf::PdmVariantEqualImpl<T>::equal(const QVariant&, const QVariant&) [with T = QString]’ at /home/builder/gitroot/ResInsight/Fwk/AppFwk/cafProjectDataModel/cafPdmCore/cafPdmFieldTraits.h:158:98,
    inlined from ‘bool caf::pdmVariantEqual(const QVariant&, const QVariant&) [with T = QString]’ at /home/builder/gitroot/ResInsight/Fwk/AppFwk/cafProjectDataModel/cafPdmCore/cafPdmFieldTraits.h:230:41,
    inlined from ‘bool caf::PdmFieldUiCap<FieldType>::isQVariantDataEqual(const QVariant&, const QVariant&) const [with FieldType = caf::PdmField<QString>]’ at /home/builder/gitroot/ResInsight/Fwk/AppFwk/cafProjectDataModel/cafPdmUiCore/cafInternalPdmUiFieldCapability.inl:286:67:
/usr/include/x86_64-linux-gnu/qt6/QtCore/qstring.h:1355:51: warning: ‘SR.8812’ may be used uninitialized [-Wmaybe-uninitialized]
 1355 | QString::QString(const QString &other) noexcept : d(other.d)
      |                                                   ^~~~~~~~~~