[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/workspacesView.js

@@ -484,7 +484,10 @@ NewWorkspaceArea.prototype = {
     },
 
     setStyle: function(isHover) {
-        this._child1.set_style_pseudo_class(isHover ? 'hover' : null);
+        if (isHover)
+            this._child1.add_style_pseudo_class('hover');
+        else
+            this._child1.remove_style_pseudo_class('hover');
     }
 };