Add support for Icedove

Previously there was no support for [Icedove](https://en.wikipedia.org/wiki/Mozilla_Corporation_software_rebranded_by_the_Debian_project), 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](https://extensions.gnome.org/extension/519/messaging-menu/)
(user Galeanthrope on 2012-12-30).

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).