Colorselector: Palette_items_get API added.

Colorselector: Palette_items_get API added.

Summary:
Summary:
Issue:Colorselector adds default palette items by reading colors
from config, application do not have any way to get that color list.
Solution: API added to get the palette's item list.
Signed-off by: Shilpa Singh(shilpa.singh@samsung.com)

Test Plan:
Test Plan: Create a list pointer and call palette_items_get API to get
the list of color items. Change the color of any item to confirm
whether the list has been returned correctly.

Eina_List *list = elm_colorselector_palette_items_get(cs);
Eina_List *list2 = eina_list_last(list);
Elm_Object_Item *color_item = eina_list_data_get(list2);
elm_colorselector_palette_item_color_set(color_item, 0, 0, 0, 255);

Reviewers: seoz, Hermet, raster

CC: govi, myoungwoon
Differential Revision: https://phab.enlightenment.org/D503