Fixed TabbedContainer sizing when tabs are not visible.

The Qt sizeHint() and minimumSizeHint() no longer include the size the tabs would need if they were visible.
Fixed vertical sizing of Timeline in default layout.

This is a bit of a hack but having spent half the day looking at it I think this is the most pragmatic approach for now - see comments for details.
Merge pull request #500 from johnhaddon/timelineHeight

Timeline height