entry: add elm_entry_select_region_get() API

entry: add elm_entry_select_region_get() API

Summary:
Already, there is a way to set a selection region:
elm_entry_select_region_set()
The get() API also useful and there is needs for this
inside of elm_entry.c. Add the API and replace codes
in atspi_text_selection_get with the API.
@feature

Test Plan:
1. Run "elementary_test -to entry3"
2. Make a selection on text.
3. Press "Sel" button.

Reviewers: tasn, herdsman, cedric, woohyun, Jaehyun, Hermet

Subscribers: Hermet

Differential Revision: https://phab.enlightenment.org/D3639