sap.ui.core.CSSSize supports css calc()

First things first: **I hereby declare to agree to the OpenUI5 Individual Contributor License Agreement.**

This PR should fix #185. 

Should I add more test cases? 

There is one little problem with the current implementation (beside the regexp beeing way to big): It doesn't check for logical errors like "3 + 10rem". 
Additionally it does not support features like attr() in calc() (yet).

I don't expect this PR to be pulled immediately, just wanted to get some input on how to get this done.