Make actions get the right object

Currently, if a comment is flagged, triggered node actions will fail to work.

The patch fixes this by:

1. Asking the flag object to provide all relevant objects (there's a new method: get_relevant_action_objects).
2. Send the correct object to the action ("the correct object" = "usually what $action->type says it is").
3. This patch also adds all the "relevant objects" to the $context. This makes it possible, e.g., for the "tokenized email" action to recognize node tokens even when the action is triggered for a comment.