Change packaging for GSS

Bundle closure stylesheet jar into gwt-user instead of gwt-dev

Change-Id: If8d2a2da0fa14388ec4b3391c186c3e09e91b8fe
Fix Java6 compatibility in UiBinder.

Due the introduction of GSS in uibinder, java6 compatibility
is broken if uibinder is used.
This change encapsulates the code using GSS in its own class
and brings back Java6 compatibility if GSS is not used.

Change-Id: I1d84b743a8723c364920aee09337305f7a959f81