input method: Use IbusIMContext "forward-key-event" signal

In js/misc/inputMethod.js, the input method needs to receive IBusIMContext 'forward-key-event' signal. This signal is often used to forward key event to the client by ibus language engine. The current version of ibus-hangul depends on this.

For example (gtk): https://github.com/ibus/ibus/blob/d784e04c38eeb069f9a8da8b30743f4463fa34c3/client/gtk2/ibusimcontext.c#L1467