add fixedChildSize api to DataList to bypass guessing for sizing and optimizing performance for fixed-size children

Enyo-DCO-1.1-Signed-Off-By: Cole Davis (cole.davis@lge.com)
Merge pull request #613 from enyojs/GF-57004-cole-davis

Fix for: GF-57004

Reviewed-By: Ben Combee (ben.combee@lge.com)