the best would be, if the TabBar widget would not base on
HorizontalPanel. a FlowPanel would be better so if it on the end of
space then it flow to the next row.
but i think it is not possibel to change this. only solution: copy the
whole TabBar and TabPanel to your own TabFlowBar and TabFlowPanel and
replace the panel = new HorizontalPanel() with the flowpanel.