Xfce

Major Xfce refactoring, also adding versions 4.10.* now into **master**

After merging I'll commit the minor change needed in NixOS https://github.com/vcunat/nixos/commit/580c0ccbf7b3c9aabbd89c286ac8071d2eef10fc.

Commented on #296 and https://github.com/NixOS/nixpkgs/commit/9abff7a85ec47613cd701c4b319fc6fe8fed4fae