Add HasText to the implemented interfaces of MValueBoxBase

This restores compatibility with GWT 2.6, where HasText came from AutoDirectionHandler.Target.

Note that this reverts 3fd1752fb34429f43637fc29c577d1a75a1d6f7a again.
Merge pull request #216 from ankon/gwt27-hastext

Add HasText to the implemented interfaces of MValueBoxBase