Support for calling D-Bus methods from scripts

A new global method is exported to perform a remote method call
through D-Bus. The call is always performed async invoking a
callback once the call completed passing through the arguments
received from D-Bus.

Possible code looks like the following:
callDBus("org.kde.kwin", "/KWin", "org.kde.KWin", "currentDesktop", function (desktop) {
print("Current Desktop through D-Bus: ", desktop);
});

REVIEW: 105396