elm_progressbar: show status when unit_format_func is set

elm_progressbar: show status when unit_format_func is set

Summary:
 - elm,state,units,visible signal is emitted only when
   unit is set, but not when unit_format_func is set.
 - Since default unit has been set, this signal is emitted
   but signal will not be emitted after unit is set to NULL.

Test Plan:
 1. Create a progressbar.
 2. elm_progressbar_unit_format_set(obj, NULL);
 3. set unit_format_func by elm_progressbar_unit_format_function_set()
    and observe elm.text.status part visible.

Reviewers: Hermet, jpeg, cedric

Reviewed By: cedric

Subscribers: D-TAU, eunue, conr2d, cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D4210

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>