[StWidget] add list-like methods for style_class and pseudo_class

Since style_class and pseudo_class are space-separated lists of names,
add new methods to add and remove individual names rather than just
re-setting the entire name.

Update existing code to use the new pseudo-class methods where
appropriate. In some cases, this may result in actors having multiple
pseudoclasses where previously they only had one at a time, but there
don't seem to be any visible differences.

(There are some places that could usefully use the new style_class
methods as well, but this patch doesn't change them.)

Also, update test-theme.c to test the new methods.

https://bugzilla.gnome.org/show_bug.cgi?id=604943

src/st/st-scroll-bar.c

@@ -723,7 +723,7 @@ handle_capture_event_cb (ClutterActor *trough,
                                                ((ClutterButtonEvent*) event)->y);
       if (target != bar->priv->handle)
         {
-          st_widget_set_style_pseudo_class ((StWidget*) bar->priv->handle, NULL);
+          st_widget_remove_style_pseudo_class ((StWidget*) bar->priv->handle, "hover");
         }