components/dbus: add SignalWatcher QML type

Reason for the change

The class creates a listener to D-Bus signals in QML.

Example:

SignalWatcher {
    busType: BusType.Session
    service: "org.kde.plasmashell"
    path: "/PlasmaShell"
    iface: "org.kde.PlasmaShell"
    function dbuswallpaperChanged(screenNum) {
        console.log(screenNum)
    }
}

Test plan

Pass dbusconnectionstest

Screenshots or screen recordings

Bugs fixed

Closes #121 (closed)