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.