GUI: Enable GWT 2.6.0

- Update GWT 2.5.1 -> 2.6.0
- Added ie10 permutation.
- Should bring better performance and stability
  to both dev-mode and runtime.

- Fixed issue in PerunAttributeCell when event.currentTarget
  was not filled by jQuery, but was required by GWT to handle
  events. So now removing value from list/map attributes in
  table correctly calls update to the attr object itself.
- Source format of PerunAttributeValueCell.
Merge pull request #261 from zlamalp/gwt26

GUI: Enable GWT 2.6.0