st: Remove st-container

At this point, StContainer is a dummy class that does nothing, so it's
safe to remove.

https://bugzilla.gnome.org/show_bug.cgi?id=670034

src/shell-stack.h

@@ -19,14 +19,14 @@ typedef struct _ShellStackPrivate ShellStackPrivate;
 
 struct _ShellStack
 {
-    StContainer parent;
+    StWidget parent;
 
     ShellStackPrivate *priv;
 };
 
 struct _ShellStackClass
 {
-    StContainerClass parent_class;
+    StWidgetClass parent_class;
 };
 
 GType shell_stack_get_type (void) G_GNUC_CONST;