Add new usercallbacks for executable to know when new webview created and destroyed.

Add new usercallbacks for executable to know when new webview created and destroyed.

[Issue#] executables that use wrt-core lib may need to know when new webview is created or destroyed.
[Bug] N/A
[Cause] Now there is no way for each executable using wrt-core to know that time
[Solution] New two UserDelegates callbacks are added, and they are called in window create, close callback of viewlogic
[Verification] Register new two UserDelegates callbacks in your executable (like wrt-client),
               and let your executable to load specific web contents that executes window.open as '_blank'

Change-Id: I40e81111ad3af115f2684a4957ece921d9a18174