listbase: Grab focus on items instead of container

listbase: Grab focus on items instead of container

The container view itself being focusable makes keyboard navigation
slower by adding a useless focus step.

It also means if an item gets removed, the focus jumps back to the view,
instead of jumping to the next item, as seen in nautilus bug report:
https://gitlab.gnome.org/GNOME/nautilus/-/issues/2489

Instead of making the GtkListBase container itself focusable, override
the .grab_focus() vfunc. This way, calling gtk_widget_grab_focus() on
the view container keeps working sucessfully, but focuses the focus
item directly instead.

This is particularly useful to have because applicaiton authors do
not have direct acess to this class's children, so they can't call
gtk_widget_grab_focus() on them directly.