ENYO-451: Add "webkitvisibilitychange" event to enyo.dispatcher.events

Enyo-DCO-1.1-Signed-off-by: Mikyung Kim <mikyung27.kim@lge.com>
ENYO-451: Added visibilitychange to support other browsers

Enyo-DCO-1.1-Signed-off-by: Mikyung Kim <mikyung27.kim@lge.com>
Merge pull request #927 from enyojs/ENYO-451-mikyungkim

ENYO-451: Add "webkitvisibilitychange" event to enyo.dispatcher.events

Reviewed By: Derek Anderson <derek.anderson@lge.com>
Integrated-By: Aaron Tam (aaron.tam@lge.com)