Added base class AIStateMachine.

This is the skeleton needed to implement classes that can be reused and
work together, which can perform asynchronous tasks (read: need to wait
for certain events before they can continue).

An example would be the task of waiting for a given inventory folder to
be read. This could then be used to improve the builtin AO
(automatically reading that folder when a notecard is dropped, and
continuing when the whole folder is read).

It's first use will be communication with a filepicker that runs
in a plugin.

doc/contributions.txt

@@ -114,6 +114,7 @@ Aleric Inglewood
 	IMP-670
 	IMP-701
 	IMP-734
+	IMP-735
 Alissa Sabre
 	VWR-81
 	VWR-83