st/entry: Allow hint actor to shrink

We currently always ignore the minimum width of the hint actor and
request/allocate the preferred width. This can be problematic with
labels with long text, where we should rather ellipsize the text
than allow the entry to grow indefinively.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2607


(cherry picked from commit 7d7a15f9)