Genlist: Add function to search item by string.

Genlist: Add function to search item by string.

Summary:
This function allows user to search for item in Genlist.
It takes four search parameters:
1. pointer to function to get text of the item. It could be the same with item's
   get_text function. This parameter is added to let user use the specific search key
   and to avoid problems with setting item's text, that is not constant.
2. pointer to the item from which search should start.
3. search pattern.
4. fnmatch() flags.
To check it's usage the new test is added to the elementary_test (Genlist Item Search By Text)

Reviewers: cedric, seoz, raster

CC: reutskiy.v.v
Differential Revision: https://phab.enlightenment.org/D566