[Engine] Add interface to fire custom js event for Tizen Webapp

[Engine] Add interface to fire custom js event for Tizen Webapp

[Issue#] There is needs that wrt provides interface to fire some custom events
[Feature] N/A
[Cause] N/A
[Solution] add new functions to RunnableWidgetObject for firing event
           and modify that plugin module of wrt requests to plugin logic of wrt-plugins-common for many custom events

Change-Id: I9db6cc54e7c462f83067b7c1cf2779a88f1d2d18