UI multi option action menu for workflow actions

* Added a popup menu for directly choosing which workflow action to take next
* Fixed a few 3.1 upgrade issues with WorkflowTransition's canExecute codepath.

Conflicts:
	code/dataobjects/WorkflowTransition.php
If the Notify action has no users assigned for it, return 'true' as we don't want it holding up the continuation of the workflow
FIX Ensure workflow options work on DataObjects

Improved the handling of workflow options fields on non-sitetree dataobjects,
particularly those edited in a gridfield
Merge pull request #160 from nyeholt/master

UI - multi option action menu for workflow actions