Add placeholderTextColor property for editors

import QtQuick 2.9
import QtQuick.Controls 2.3

ApplicationWindow {
    visible: true
    width: 640
    height: 480

    TextField {
        anchors.centerIn: parent
        color: "white"
        placeholderText: "Search..."
        placeholderTextColor: "#aabbccdd" // <===
        background: Rectangle {
            color: "black"
        }
    }
}