Added AIFetchInventoryFolder and more.

Added a new statemachine AIFetchInventoryFolder, which can be used
to fetch the contents of a folder by name or UUID.

Also added AIEvent (and one event,
AIEvent::LLInventoryModel_mIsAgentInvUsable_true, which is needed
for AIFetchInventoryFolder).

Fixed LLInventoryModel::sBackgroundFetchActive to correctly reflect
whether or not LLInventoryModel::backgroundFetch is added to
gIdleCallbacks.

Avoid duplicated entries in sFetchQueue.

Reset sFullFetchStarted in LLInventoryModel::stopBackgroundFetch to
allow for a renewed full fetch when some single-folder fetch stops it.

Added AIStateMachine::mQueued to make calling 'cont()' more robust:
calling cont() / idle() / cont() on a row would otherwise add a
statemachine twice to the active list, which would cause a crash
when it's killed.

indra/llcommon/aithreadsafe.h

@@ -403,7 +403,8 @@ protected:
 	// Locking control.
 	LLMutex mMutex;
 
-	// For use by AIThreadSafeSimpleDC
+	friend struct AIRegisteredStateMachinesList;
+	// For use by AIThreadSafeSimpleDC and AIRegisteredStateMachinesList.
 	AIThreadSafeSimple(void) { }
 	AIThreadSafeSimple(AIAPRPool& parent) : mMutex(parent) { }