listview: Add GtkListItem::focusable property

listview: Add GtkListItem::focusable property

This makes the question if a listitem can be focused or not an explicit
decision by application developers.

Previously an item could be focused if it was selectable and no child or
grandchild was focusable - so if you put a label and icon into it, the
item was focusable, but if you put a GtkTreeExpander or a GtkButton into
it, the item wasn't. This needs to be decided explicitly now.

Technically this is an API break, because the previous behavior does not
exist anymore.
But I really don't want to make this a tristate (focusable, not
focusable, automatic), because then binding it to other things gets
hard, and because all the other focusable proeprties are booleans, too,
and working with them gets a lot harder.

Related: #3910