genlist: add reusable_content_get item class function for support automatical content reusing in genlist.
Summary:
Add new Item Class function reusable_content_get for improve genlist
performance.
If user want to reuse specific part contents, return old content
which was passed by parameter of rreusable_content_get function.
genlist will automatically caching those contents when item unrealized,
and reusing it on item realizing.
reusable_content_get will be performed content_get with old object caching.
But if user return NULL in reusable_content_get, content_get is called to
get new object.
This feature is refered in Task T2813
@feature
Test Plan: add test in genlist 1 case.
Reviewers: raster, singh.amitesh, cedric
Reviewed By: cedric
Subscribers: smohanty, seoz, woohyun, Hermet, eagleeye
Differential Revision: https://phab.enlightenment.org/D3252
Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>