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

js/ui/main.js

@@ -77,7 +77,7 @@ function start() {
         children[i].destroy();
 
     let style = St.Style.get_default();
-    let stylesheetPath = global.datadir + "/gnome-shell.css";
+    let stylesheetPath = global.datadir + "/theme/gnome-shell.css";
     style.load_from_file(stylesheetPath);
 
     global.connect('panel-run-dialog', function(panel) {