Fix PC3.GroupBox.color

SystemPaletteSingleton was a QQC1.Private import. Not sure why it's in a QQC2 theme. Nothing in Plasma uses this GroupBox so it makes sense why it wasn't noticed before.

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Note: I am not sure if import "." as PC3 is the proper way to import PC3.Label in a QQC2 style.

This fixes this error:

file:///usr/lib/qt/qml/org/kde/plasma/components.3/GroupBox.qml:30: ReferenceError: SystemPaletteSingleton is not defined

Code was written 4 years ago. Since it's "SystemPallete", it probably displays the App/Config text color, not the panel color. It currently draws a black text on grey bg with breeze-dark.

• https://github.com/qt/qtquickcontrols/blob/dev/src/controls/Private/SystemPaletteSingleton.qml
• https://invent.kde.org/frameworks/plasma-framework/-/blame/master/src/declarativeimports/plasmacomponents3/GroupBox.qml

To test, just edit: /usr/lib/qt/qml/org/kde/plasma/components.3/GroupBox.qml

Then create a simple helloworld widget with metadata.desktop from the wiki and this contents/ui/main.qml.

• https://develop.kde.org/docs/plasma/widget/setup/#metadatadesktop

// contents/ui/main.qml
import QtQuick 2.0
import QtQuick.Controls 2.0 as QQC2
import QtQuick.Layouts 1.0
import org.kde.plasma.core 2.0 as PlasmaCore
import org.kde.plasma.components 3.0 as PlasmaComponents3
import org.kde.plasma.plasmoid 2.0

Item {
    Plasmoid.fullRepresentation: ColumnLayout {
        QQC2.GroupBox {
            title: i18n("Enabled")
            Item { implicitWidth: 480 }
        }
        PlasmaComponents3.GroupBox {
            title: i18n("Enabled")
            Item { implicitWidth: 480 }
        }
        PlasmaComponents3.GroupBox {
            id: groupBox1
            title: i18n("Enabled")
            label: PlasmaComponents3.Label {
                x: groupBox2.leftPadding
                text: groupBox1.title
                enabled: groupBox1.enabled
                color: PlasmaCore.Theme.textColor
            }
            Item { implicitWidth: 480 }
        }
        QQC2.GroupBox {
            enabled: false
            title: i18n("Disabled")
            Item { implicitWidth: 480 }
        }
        PlasmaComponents3.GroupBox {
            enabled: false
            title: i18n("Disabled")
            Item { implicitWidth: 480 }
        }
        PlasmaComponents3.GroupBox {
            id: groupBox2
            enabled: false
            title: i18n("Disabled")
            label: PlasmaComponents3.Label {
                x: groupBox2.leftPadding
                text: groupBox2.title
                enabled: groupBox2.enabled
                color: PlasmaCore.Theme.textColor
            }
            Item { implicitWidth: 480 }
        }
    }
}