Fix installation and distribution of stylesheet data

Install and distribute gnome-shell.css and theme images. They are moved
down from $datadir to $datadir/theme to avoid a weirdness where we have
images in $datadir and then also in $datadir/images.

(Also moved in the source tree to avoid adding another difference between
installed and uninstalled operation.)

https://bugzilla.gnome.org/show_bug.cgi?id=595989