st/entry: Fix primary/secondary icon leak

st/entry: Fix primary/secondary icon leak

_st_entry_set_icon() takes a ref when setting the primary/secondary
icon, but the ref is not removed when unsetting the icon or the entry is
destroyed.

Since clutter_actor_add_child() already adds a ref to the icon, there is
actually no need to manually add another ref. So stop doing that to fix
this leak.

This however also requires that StPasswordEntry takes ownership of its
own icon that it may set or unset multiple times. Since it is initially
floating we need to sink the ref for that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4095>