Use gimp_item_is_content_locked() instead of gimp_item_get_lock_content()

Use the new API whenever we want to determine the item's effective
lock state (whether we can write to the item's content or not). Use
gimp_item_get_lock_content() only in code that actually deals with
*this* item's locked state, which is only the PDB wrappers and GUI to
modify the flag on the item itself.

app/actions/select-actions.c

@@ -146,7 +146,7 @@ select_actions_update (GimpActionGroup *group,
 
       if (drawable)
         {
-          writable = ! gimp_item_get_lock_content (GIMP_ITEM (drawable));
+          writable = ! gimp_item_is_content_locked (GIMP_ITEM (drawable));
 
           if (gimp_viewable_get_children (GIMP_VIEWABLE (drawable)))
             children = TRUE;