Instead of requiring sassc to be installed add meson subprojects
which build libsass and sassc (currently both forks of mine, tested
under linux/mingw/msvc) when needed.
This allows us to drop the generated .css files and build scripts from git.
See #1502
This subdirectory gets in the way when integrating the inspector
build more fully with GTK+, and does not really add anything.
Just move everything one level up.