Use kali-wallpapers-2020.4 to store the new wallpapers

Keep the "default" symlink for XFCE in the kali-themes-common package.
We need to embed the version in the package name if we want the old
wallpapers to coexist with the new ones on user's systems to avoid
breaking their non-default backgrounds.

bin/update-default-backgrounds

@@ -7,7 +7,7 @@ LOGIN_BACKGROUND=$2
 # xfce background
 if [ -e wallpapers/backgrounds/kali-16x9/$DESKTOP_BACKGROUND.png ]; then
     mv -v wallpapers/backgrounds/kali-16x9/$DESKTOP_BACKGROUND.png \
-	wallpapers/backgrounds/kali-16x9/default
+	share/backgrounds/kali-16x9/default
 fi
 
 # login/lockscreen

debian/control

@@ -12,7 +12,7 @@ Vcs-Browser: https://gitlab.com/kalilinux/packages/kali-themes
 
 Package: kali-themes-common
 Architecture: all
-Depends:  kali-wallpapers (= ${source:Version}),
+Depends:  kali-wallpapers-2020.4 (= ${source:Version}),
  ${misc:Depends}
 Suggests: gtk2-engines-murrine
 Breaks: gnome-theme-kali (<< 2019.4), desktop-base (<< 10.0.3+kali2)
@@ -60,12 +60,23 @@ Description: Configure all desktops to use the Kali theme
  provides configuration files and other settings for each desktop so that
  they use the Kali theme by default.
 
-Package: kali-wallpapers
+# The package is versioned so that it stays around on end-user systems whenever
+# we introduce a new set of wallpapers, thus not breaking the user's background
+# if he selected a non-default wallpaper. The binary package can possibly be
+# moved to kali-legacy-wallpapers.
+#
+# Note: On next version bump, we can get rid of the breaks/replaces.
+Package: kali-wallpapers-2020.4
 Architecture: all
 Depends: ${misc:Depends}
 Breaks: kali-themes-common (<< 2020.4.3)
-Description: Default wallpapers for kali
- This package contains multiple wallpapers for Kali.
+Replaces: kali-themes-common (<< 2020.4.3)
+Description: Default wallpapers for Kali Linux
+ This package contains multiple wallpapers for Kali Linux.
+ .
+ If you have multiple kali-wallpapers-YYYY.X packages on your system, you can
+ remove them all except the latest which is a dependency of kali-themes-common
+ and is thus required.
 
 Package: gnome-theme-kali
 Architecture: all