genlist: add SIG_CHANGED("changed") smart callback in genlist.

genlist: add SIG_CHANGED("changed") smart callback in genlist.

Summary:
SIG_CAHANGED notify user to
genlist is now changed their items and properties and all
calculation is finished, so user can get correct values in
that callback.

This SIG_CHANGED smart callback already supported in gengrid.

Test Plan:
N/A
This patch is addition of new smart callback.

Reviewers: raster, seoz, singh.amitesh

Subscribers: Hermet, cedric, eagleeye

Differential Revision: https://phab.enlightenment.org/D2773

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>