The group role that we've used before has some
implications of semantic grouping, whereas these
containers are mainly about layout, so use the
generic role instead.

This should not affect the translation to AT-SPI
at all.

The affected containers are: box, grid, centerbox,
scrolledwindow, viewport, windowhandle, aspectframe.

The role of GtkTreeExpander has been changed to
button instead, since it acts as a button.