edje: make functions return Eina_Bool and fix documentation.

edje: make functions return Eina_Bool and fix documentation.

Make the following functions return Eina_Bool so the caller can detect errors:
edje_edit_part_drag_x_set
edje_edit_part_drag_y_set
edje_edit_part_drag_step_x_set
edje_edit_part_drag_step_y_set
edje_edit_part_drag_count_x_set
edje_edit_part_drag_count_y_set

Added functions for threshold:
edje_edit_part_drag_threshold_set
edje_edit_part_drag_threshold_get

Fixed description of 'count' parameter of dragable.x/y

Reviewers: cedric, seoz

Reviewed By: cedric

CC: reutskiy.v.v
Differential Revision: https://phab.enlightenment.org/D304

Signed-off-by: Cedric Bail <cedric.bail@samsung.com>