components: add DBusServiceWatcher

This is part of the effort to remove the executable dataengine. This was originally planned to be used in the task manager to provide information about the availability of the window view effect, but custom widgets can also benefit from the component.

Given that https://bugreports.qt.io/browse/QTBUG-90785 is unlikely to be implemented in the near future, I'd like to implement some Plasma's own DBus QML types.

Example

import org.kde.plasma.workspace.dbus as DBus

DBus.DBusServiceWatcher {
    id: watcher
    busType: DBus.BusType.Session
    watchedService: "org.kde.KWin.Effect.WindowView1"
}

registered

Once the idea is accepted, I will also add a test.