Make tab contents expand to fill tab widget size.
Make tab contents expand to fill tab widget size.

Fixes #1243
SylvainCorlay/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets