When using the meson build system, gvc will be built as a subproject.
As meson restricts submodules to a subprojects/ directory in the
source tree root, move the submodule there.
https://bugzilla.gnome.org/show_bug.cgi?id=783229
The sass sources now live in a project in GNOME, so they can
be used in multiple projects like gnome-shell-extensions.
Because of that, add gnome-shell-sass as a submodule of gnome-shell
and remove the sass sources from the theme directory.