When using keynav in the category views, the fact that the categories are
separate icon views leads to confusion, because each category will end up
with its own selected icon, so that you can end up with multiple selected
items (and the difference between focused/non-focused is not visually
obvious here). Also, since the category views appear visually as a single
unit, it is irritating that arrow keynav stops at category borders.
This commit arranges things so that only one category will ever have a
selected item by unselecting on focus-out. Furthermore, it uses the
keynav-failed signal and some new GTK+ api to make arrow up/down work across
category borders, in the expected way.
e14803b391