Genlist Item Pin Feature

Genlist Item Pin Feature

Summary:
**@feature** T6241

This feature enables genlist to pin an item to viewport which will
be available always for user to view/select.

**Use Case**:
In a big list of music, most times when user finds a song which they
like, before playing that they may want to go through the entire list
to check whether there is some other good songs, but
after seeing the entire list user have to again scroll back to the
position of item which they liked to play it then.
In this case item pinning can be used, so that the item
which they want to keep for future selection can be pinned
and then it will remain in viewport, finally when user want to do
operation on item, it will be readily available in viewport.

Signed-off-by: Godly T.Alias <godlytalias@yahoo.co.in>
Test Plan: Elementary Test -> Genlist -> Double click on items to enable/disable pinning

Reviewers: raster, cedric, prince.dubey, SanghyeonLee

Subscribers: rajeshps, jpeg, shilpasingh

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D5340