Add specific interface for support of custom js event

Add specific interface for support of custom js event

[Issue#] Requirement for custom js event should be applied to WRT
[Bug] N/A
[Cause] N/A
[Solution] New interface is created for registering custom js event internally and firing the js event to web contents on specific timing.
           Especially this interface is needed from custom js event regarding ime show/hide including existing appservice event.
[Verification] register "imeshow" js event on your javascipt source using addEventListener()
               and then use input tag of 'text' type to show ime or hide it

Change-Id: Ia14867d13b3569b598d69e558db7828876439ef0