elementary label: use text width when label calculates duration for slide

elementary label: use text width when label calculates duration for slide

Summary:
When developers customize theme for making different type of sliding label,
it would be better to send a duration based on whole text.
[The whole text width - label object's w] is only useful for short, bounce
type sliding.

Test Plan: N/A

Reviewers: raster, tasn, herdsman, cedric

Reviewed By: cedric

Subscribers: minkyu, akanad, z-wony, Blackmole, jpeg

Differential Revision: https://phab.enlightenment.org/D4255

Change-Id: I92e0a4bca43e456b92cba9654a13b59ac138a907