[resend] freeze break: confirmation dialog for extension installation

[Owen Taylor <otaylor redhat com>]

[ sent to gnome-18n last time, resend with the correct address ]

As part of the work for extensions.gnome.org we've added a way to have
the extensions.gnome.org website trigger downloading and installing
an extension via a browser plugin. (The browser plugin and the download
are locked to extension.gnome.org.)

One thing that I pointed out in review was that I thought it was
important to have an obviously client-side dialog before downloading and
installing an extension - to make it clear to the user what is going on,
and catch any funny business is going on.

(E.g., XSS exploit on shell.gnome.org allows someone to script the
plugin to download an extension they've snuck past code review.)

https://mail.gnome.org/archives/desktop-devel-list/2011-August/msg00242.html

Adding this dialog requires a string freeze break to add the strings:
 
  Download and install '%s' from extensions.gnome.org?
  Install

(First string still under discussion, but will be something like that.)

Current patch on: https://bugzilla.gnome.org/show_bug.cgi?id=658612

Thanks!

- Owen