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-generic-container.h

@@ -29,14 +29,14 @@ typedef struct _ShellGenericContainerPrivate ShellGenericContainerPrivate;
 
 struct _ShellGenericContainer
 {
-    StContainer parent;
+    StWidget parent;
 
     ShellGenericContainerPrivate *priv;
 };
 
 struct _ShellGenericContainerClass
 {
-    StContainerClass parent_class;
+    StWidgetClass parent_class;
 };
 
 GType    shell_generic_container_get_type         (void) G_GNUC_CONST;