Pass selected tab rect to style

In particular QGtkStyle depends on having the selected tab rect
in the style option. Otherwise it cannot draw the tab frame
correctly as we pass this information on to gtk.

In addition we were not correctly setting the tab position on
the frame.

Change-Id: I9aa17d6b33f6b283122014994dc92a6afb913f30
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>