Allow dynamic/flexible/freely definable transitions per state

Problem/Motivation

Is it possible to freely define the allowed transitions on a given state?
This is not intended as a guard condition. On the contrary, it is meant to open up new transitions, based on conditions that the core workflow module cannot verify since it restricts itself to roles-based transitions.

This patch opens up the possibility for 3rd party modules to create their own data-driven transitions based on other criteria. For example, one such module would alter allowed state transition per user. Custom modules can freely define the reasoning behind such rules.

API changes

A new hook 'hook_workflow_permitted_state_transitions_alter()' is created, allowing to add or remove allowed transitions from/to the proposed list. The list is used to generate the options in the Workflow widget. See file workflow.api.php for more info.

Original report by [username]

In my current usage of the workflow, I need to let a *specific user* execute a transition, based on a user reference field in the node instead of a user role. I also ran into other scenarios that are more indirect. I thus came up with a generic mechanism to allow modules to specific dynamic transitions, given a state, a user account, and a node. This patch is attached.

The way it works is by calling 2 new hooks at the relevant places. hook_workflow_transitions_alter alters the current allowable transitions to include new ones or stop existing ones. hook_workflow_transitions_info lists the modules that will intercept the transitions to display them on the admin transition grid.

A typical usage of these hooks would be thus:

<?php
// Define the sids we care about.
define('WF_STATE_1', 1);
define('WF_STATE_2', 2);
define('WF_STATE_3', 3);

function mymodule_workflow_transitions_info() {
  return array(
    WF_STATE_3 => array(
      WF_STATE_2 => t('Let the admin go back to state 2'),
      WF_STATE_1 => t('Let the referenced user go back to state 1'),
    ),
  );
}

function mymodule_workflow_transitions_alter(&$transitions, $sid, $account, $node) {
  if ($sid == WF_STATE_3) {
    if ($account->uid == 1) {
      $transitions[WF_STATE_2] = workflow_get_state_name(WF_STATE_2);
    }
    if ($node->field_user[0]['uid'] == $account->uid) {
      $transitions[WF_STATE_1] = workflow_get_state_name(WF_STATE_1);
    }
  }
}

?>

Thanks for your consideration. I'll be glad to enhance and polish this patch if needed.