list box: Make focusable headers possible

list box: Make focusable headers possible

Previously we were assuming that only list box rows could occur
as focus children of a list box, and would crash if that wasn't
the case. This commit handles this case, and integrates focusable
headers into directional keynav and the focus chain.

The typical case of using separators as headers is not affected
by this change.

https://bugzilla.gnome.org/show_bug.cgi?id=753694