Introduce fileselector interface and applied to elm_fileselector.

Introduce fileselector interface and applied to elm_fileselector.

Summary:
fileselector_button and fileselector_entry should have almost same interfaces with fileselector.
Previously , we exposed them individually such as elm_fileselector_is_save_set().

Instead of adding the same APIs three times, we can reuse one fileselector's API using Eo interface.

This patch introduced fileselector interface and applied it to elm_fileselector.

Reviewers: seoz, raster

Reviewed By: raster

Differential Revision: https://phab.enlightenment.org/D475