listbox: Avoid using show_all on rows

listbox: Avoid using show_all on rows

Don't use gtk_widget_show_all() on row widgets because that would
unconditionally show all of its children. This might be unwanted in case
the row implementation wants to keep some of its children hidden.

This commit changes it to use show() instead of show_all() and relies on
the row widget to control the visibility of its children itself as
appropriate.

https://bugzilla.gnome.org/show_bug.cgi?id=753392