gtk-demo: Use explicit resource:// urls in CSS

... because we load the data from a GtkTreeView::changed callback, the
relative directory is cwd. And we want the resources...

demos/gtk-demo/css_accordion.css

@@ -1,4 +1,4 @@
-@import url("reset.css");
+@import url("resource:///reset.css");
 
 * {
     transition-property: color, background-color, border-color, background-image, padding, border-width;
@@ -49,4 +49,4 @@ GtkWindow {
 .button:hover:active,
 .button:active {
     background-color: #993401;
-}
+}