Extend the Act system

[mostafa]

This is an epic ticket to manage Act system development:

• Add a way to cancel actions #463
• Enrich policy input #462
• Add queueing for async actions #464
• Add distributed runner for running actions #472
• Create helper functions to make writing policies easier #541
• Refactor notification hooks #397
• Make actions extensible via plugins and/or a scripting language #467
• Enable signals, policies and actions for all hook functions #468
• Implement triggers (chain of actions) #469
• Integrate with policy engines #470
• Investigate other signal sources #471