Label: add 2 new api for setting the slide animation speed.

Label: add 2 new api for setting the slide animation speed.

Added API:
void elm_label_slide_speed_set(Evas_Object *obj, double speed);
double elm_label_slide_speed_get(const Evas_Object *obj);

This work is based on the previous reverted commit by nirajkr, thanks!
I hope this will fullfill his requirement, please let me know of
any problems.