ecore: add an index property on Efl.Model_Composite and handle children_slice_get.

ecore: add an index property on Efl.Model_Composite and handle children_slice_get.

This will enable inheriting class to not have to implement as much code. This patch
fix also all class that use Efl.Model_Composite and its test.

Reviewed-by: SangHyeon Jade Lee <sh10233.lee@samsung.com>
Differential Revision: https://phab.enlightenment.org/D7654