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.
