Draft: applets/kimpanel: reduce allocations in `XkbLayoutManager::getLayout`

Before: 5,777 instruction reads per iteration (total: 5,777, iterations: 1

After: 1,341 instruction reads per iteration (total: 1,341, iterations: 1)

Benchmark

#pragma GCC push_options
#pragma GCC optimize("O2")

#include <ranges>

#include <QTest>
#include <QString>
#include <QProcess>

using namespace Qt::StringLiterals;

class StringFamilyTest : public QObject {
    Q_OBJECT
private Q_SLOTS:
    void initTestCase();
    void test_splitQByteArray();
    void test_split_view_const();
private:
    QByteArray output;
};

QTEST_GUILESS_MAIN(StringFamilyTest)

void StringFamilyTest::initTestCase()
{
    QProcess process;
    process.start(u"setxkbmap"_s, QStringList(u"-query"_s));
    process.waitForFinished();
    output = process.readAllStandardOutput();
}

void StringFamilyTest::test_splitQByteArray()
{
    QString m_defaultLayout;
    QString m_defaultVariant;
    QString m_defaultOption;
    QBENCHMARK {
        const QList<QByteArray> lines = output.split('\n');
        for (const QByteArray &line : lines) {
            QByteArray element("layout:");
            if (line.startsWith(element)) {
                m_defaultLayout = QString::fromLatin1(line.mid(element.length())).trimmed();
            }

            element = "variant:";
            if (line.startsWith(element)) {
                m_defaultVariant = QString::fromLatin1(line.mid(element.length())).trimmed();
            }

            element = "options:";
            if (line.startsWith(element)) {
                m_defaultOption = QString::fromLatin1(line.mid(element.length())).trimmed();
            }
            }
    }
}

void StringFamilyTest::test_split_view_const()
{
    QString m_defaultLayout;
    QString m_defaultVariant;
    QString m_defaultOption;
    QBENCHMARK {
        for (const QByteArrayView line : std::views::split(std::as_const(output), '\n')) {
            constexpr QByteArrayView element("layout:");
            if (line.startsWith(element)) {
                m_defaultLayout = QString::fromLatin1(line.sliced(element.length())).trimmed();
            }

            constexpr QByteArrayView element2("variant:");
            if (line.startsWith(element2)) {
                m_defaultVariant = QString::fromLatin1(line.sliced(element2.length())).trimmed();
            }

            constexpr QByteArrayView element3 = "options:";
            if (line.startsWith(element3)) {
                m_defaultOption = QString::fromLatin1(line.sliced(element3.length())).trimmed();
            }
        }
    }
}

#include "main.moc"

#pragma GCC pop_options