Add titlebar colors to Kirigami::PlatformTheme

Summary: This is a prerequisite for implementing the tools area in QML applications, which requires having access to the titlbar colour.

Test Plan:

import QtQuick 2.0
import org.kde.kirigami 2.12 as Kirigami

Rectangle {
    color: Kirigami.Theme.titlebarBackgroundColor
}

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: mart, plasma-devel

Tags: Kirigami

Maniphest Tasks: T10201

Differential Revision: https://phabricator.kde.org/D28949