[PC3 ToolButton] Have the label take into account complementary color schemes

Summary:
We've ported the SDDM theme to PC3 and now have black labels in PC3 ToolButtons (keyboard and session button).

The SDDM theme uses a complementary color scheme, which is something the PC2 ToolButton respects and turns the labels white.

Therefore I just copy pasted PC2's label color code to its PC3 counterpart.

BUG: 414929
FIXED-IN: 5.66

Test Plan:

import QtQuick 2.13
import org.kde.plasma.components 3.0 as PlasmaComponents
import org.kde.plasma.core 2.0 as PlasmaCore

PlasmaCore.ColorScope {
    colorGroup: PlasmaCore.Theme.ComplementaryColorGroup

    Rectangle {
        height: 80
        width: 100
        color: "red"

        PlasmaComponents.ToolButton{
            anchors.fill: parent
            text: "sup"
        }
    }
}

The label was white instead of black.

Reviewers: Plasma, ngraham, davidedmundson

Reviewed By: Plasma, ngraham, davidedmundson

Subscribers: mart, davidedmundson, fvogt, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25699