Replace the Claws Mail launcher by Icedove

To mark the deprecation of Claws Mail in 1.8 we should replace the top bar launcher with the one of Icedove.

Feature Branch: kytv/feature/10739-add-icedove-to-launcher

Original created by @sajolida on 10739 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information