add support for icedove

Previously there was no support for Icedove, which is the rebranded
Debian version of the Thunderbird e-mail client, and support for this
client has been requested on the GNOME Shell extensions website.

Using the existing code supporting Thunderbird I added code to support
Icedove (mostly just copy, paste, rename) and then tested all Icedove
menu items with GNOME Shell 3.4.2 in Debian GNU/Linux 7.0 (wheezy).