GUI: bumped to GWT 2.6 release

- Updated GUI modules definitions and pom.xml dependency.

GWT 2.6 highlights:

- Java 7 source level is default.
- Fixed memory leakage in IE and when using FlexTable
  and DialogBox widgets (we use them a lot).
- Separate permutation for IE10.
- IE6-7 and Opera disabled by default (but we can still use it,
  it will be removed in next major release).
- Fixed memory leakage in development mode & firefox, which was
  a nightmare in last two stable releases.
Add new constructor for WrongReferenceAttributeValueException

- new primary and secondary reference attribute holder
Merge pull request #173 from stavamichal/new-constructor-for-attribute-reference-exception

Add new constructor for WrongReferenceAttributeValueException
Merge pull request #164 from zlamalp/gwt26

GUI: bumped to GWT 2.6 release