genlist: add reusable_content_get item class function for support automatical content...

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>