[M108 Migration] Add "custom,scroll,begin","custom,scroll,end" support

[M108 Migration] Add "custom,scroll,begin","custom,scroll,end" support

Handles and context menu should be temporarily hidden durnig scrolling.
But specific applications like email-app uses their own scrolling
mechanism within chromium-efl does not know about this happening.
This breaks chromium-EFL behaviour.

To meet these requirements email-app provided smart callbacks to
notify engine that custom scrolling is started and finished.

The patch is a counterpart of above smart callbacks.

Reference: https://review.tizen.org/gerrit/281741

Change-Id: I5ab6d978b50b4e9ed4302ad28fcc65f1c248453b
Signed-off-by: Ayush Kumar <ayush.k123@samsung.com>