Add a minimal installation for Ubuntu Studio

Ubuntu Studio wants to add a minimal installation. The individual tasks
are metapackages that can be installed by the ubuntustudio-desktop task.

With that in mind, we would like to reintroduce
ubuntustudio-desktop-core as a minimal installation. This is made much
easier with the layered images compared to the package removal format
used by ubiquity. This also means ubuntustudio-desktop-core becomes the
base seed.

If I'm missing anything, please advise.

live-build/auto/config

@@ -953,7 +953,10 @@ case $PROJECT in
 	ubuntustudio)
 		# By default Ubuntu Studio now ships the new installer.
 		UCFLAVOUR="Ubuntu Studio"
-		STANDARD_TASKS="ubuntustudio-desktop ubuntustudio-audio ubuntustudio-graphics ubuntustudio-video ubuntustudio-photography"
+		HAS_MINIMAL=yes
+		MINIMAL_TASKS=ubuntustudio-desktop-core
+		MINIMAL_DESC="A minimal installation of $UCFLAVOUR. Customize afterwards with $UCFLAVOUR Installer."
+		STANDARD_DESC="A full installation of $UCFLAVOUR."
 		KERNEL_FLAVOURS=generic
 		do_layered_desktop_image
 		;;