gnome-shell-build-setup.sh: Move default directory to ~/gnome

gnome-shell-build-setup.sh is generally useful for working on GNOME.
If moving on from hacking on gnome-shell to some other module,
having the checkout location be ~/gnome-shell is a little odd and
cumbersome, so start out checking things out into ~/gnome/source
and installing them into ~/gnome/install.

Add a warning if the old ~/gnome-shell exists to avoid unnecessary
checking out of every module again.

https://bugzilla.gnome.org/show_bug.cgi?id=685355