frontend: Upgrade GWT to version 2.6.1

Also upgraded:
- GIN to version 2.1.2 (needed for GWT 2.6)
- GWT-P to version 1.3 (features support for GWT 2.6)
- maven.source.version for common and frontend projects to Java 7
- AspectJ to version 1.8.2 for emmiting Java 7 compatible bytecode

Removed workaround for issue not present in GWT 2.6 in SearchPanelView

GWTP upgrade required:
s/import com.gwtplatform.mvp.{client,shared}.proxy.TokenFormatter;/
s/import com.gwtplatform.mvp.{client,shared}.proxy.PlaceRequest;/
install(new CommonGinModule());

Change-Id: I76c29897960e2b61ead8176f896e45f9d7b9b576
Signed-off-by: Martin Betak <mbetak@redhat.com>