control-center network panel freeze break request

[Matthias Clasen <matthias clasen gmail com>]

We still need to land a few more last-minute features to make the
networking story palatable for 3.0.
Here is one. It adds a padlock icon to the combobox for 'secure' access points.

https://bugzilla.gnome.org/show_bug.cgi?id=645681

I'd like to commit this for .93

It uses an icon that was added to gnome-icon-theme-symbolic yesterday
for this very purpose.
No string changes.