Make desktop-base theme a symlink tree to real files

Merge the grub background files in the Grub-Theme directory.

debian/kali-themes-common.install

@@ -5,7 +5,7 @@ Icon-Theme/* usr/share/icons/
 Window-Theme/* usr/share/themes/
 Grub-Theme/* boot/grub/themes/
 Plymouth-Theme/* usr/share/plymouth/themes/
-desktop-base/* usr/share/desktop-base/
+desktop-base/* usr/share/desktop-base/kali-theme/
 kali-logos/* usr/images/kali-logos/
 xfce/xsettings.xml usr/share/kali-themes-common/
 # Those are created by "make install"