[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

js/ui/genericDisplay.js

@@ -105,7 +105,10 @@ GenericDisplayItem.prototype = {
     // Highlights the item by setting a different background color than the default 
     // if isSelected is true, removes the highlighting otherwise.
     markSelected: function(isSelected) {
-        this.actor.set_style_pseudo_class(isSelected ? "selected" : null);
+        if (iSelected)
+            this.actor.add_style_pseudo_class('selected');
+        else
+            this.actor.remove_style_pseudo_class('selected');
     },
 
     /*