Improve handling of $action object throughout Drupal core

This issue is about not optimal code patterns being used in the actions system. I'll try to make a little introduction to the issue.
Certain actions can have parameters, which are stored as serialized data in a "parameters" field in the table.
When dealing with actions, the current approach in core is to directly select actions from the actions table and deal with the rows directly. This means manually unserializing the "parameters" field each time you get a row from the actions table.

Current code also provides an actions_load($aid) function which consists of just a db_fetch_object/db_query call. This function is used only once in all the core codebase.

This patch suggests the introduction of a new function called actions_unpack($action) which takes an $action database object and unserializes the parameters field. It also corrects the code in actions.inc and system.module to conform to this new change.

The pro's for this patch are:
* a little cleaner code throughout core;
* the actions_unpack function could be easily extended with a hook to be able to manipulate actions parameters and for example, make them language-aware (see #340657: Save actions settings by locale for details)