eina: add eina_list_data_idx().

eina: add eina_list_data_idx().

Summary:
Now the developer has to iterate the whole list to find the
index of the first occurence of the data.
I see that it is possible to get the index of the item for
the genlist widget, but for the eina_list not.

With these APIs it will be easier to implement *index_get
functions for the rest of widgets which contain items (Elm_List,
Elm_Ctxpopup etc.).

These functions returns the index of the given data or node in the eina_list.

Reviewers: Hermet, cedric

Subscribers: raster, cedric

Differential Revision: https://phab.enlightenment.org/D2189

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