shellEntry: Don't show "Show Password" menu if entry icon is disabled

shellEntry: Don't show "Show Password" menu if entry icon is disabled

Right now the Show Password menu is shown unconditionally for password
entries, ignoring the org.gnome.desktop.lockdown disable-show-password
key.

StShellEntry now honors that key for its peek icon, and there's little
reason for the peek icon and the menu to ever be out of sync.

This commit forces the menu and the icon to always be in sync, and
so makes the menu lock down work automatically, too.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/687>