Consider adding a warning when starting Claws Mail once it's deprecated

Probably in 1.8, Icedove will be advertized as the default email client. By then, what about adding a warning when starting Claws Mail about it’s future removal. The message could be:

<em>Claws Mail</em> will be removed in Tails 1.9 (2016-01-26).

If you have emails saved in <em>Claws Mail</em>, you should [migrate your data to <em>Icedove</em>](https://tails.boum.org/doc/anonymous_internet/claws_mail_to_icedove].

[Start <em>Claws Mail</em> anyway] [Exit]

Parent Task: #7773 (closed)

Original created by @sajolida on 10459 (Redmine)