elementary entry: update focus state when editable state is changed

elementary entry: update focus state when editable state is changed

Summary:
To recieve keyboard events, the entry_edje should have Evas focus.
But, if a non editable Entry widget takes focus, it can't recieve
keyboard events even if it becomes editable after taking focus.
So, elm_entry_editable_set() function should update Entry's focus state.
@fix

Test Plan:
The code of elementary_test - entry is modified to test this issue.
Please, check the issue with the following steps.

1. Run "elementary_test entry"
2. Click "Unfocus" button to make entry to "unfocused" state.
3. Click "Edit" button to make entry to non-editable mode.
4. Click "Focus" button to make entry to "focused" state.
5. Click "Edit" button to make entry to editable mode.
6. See a cursor is blinking in entry.
   => But, you can't edit text without this patch.

Reviewers: raster, herdsman, cedric, jpeg, woohyun

Differential Revision: https://phab.enlightenment.org/D4858