Add many XFCE4-Apps

[schmensch]

Even though XFCE itelf isn't wayland-native yet, all apps I tested are. This includes:

• Thunar (file manager)
• XFCE4-Terminal (terminal emulator, it's in the name)
• Ristrotto (Image viewer)
• Xarchiver (Archive manager)

[mpsq]

Closing -- XFCE is a desktop environment, once most of its apps are ready, we will add XFCE directly to the corresponding section.