Turned is* functions into controls, since they have to be able to test errors, and only controls can do that, not functions.
Polished display of errors in cells and in expression preview dialog.
git-svn-id: http://google-refine.googlecode.com/svn/trunk@155 7d457c2a-affb-35e4-300a-418c747d4874
Allow expressions to be evaluated on null cells too.
Various minor UI polishing touches and minor bug fixes.
git-svn-id: http://google-refine.googlecode.com/svn/trunk@58 7d457c2a-affb-35e4-300a-418c747d4874
