There were a few "ambiguous operator new/delete" errors that were due to
inheriting from a private base class that used the leak detector. These
errors are resolved by adding the leak detector to the derived classes.
JUCE_API was missing from a few useful types, notably the ARA hosting
types.
When extra guarantees were added to ListenerList to guard against mutations of
the list from callbacks, it also resulted in removing a previously observable
guarantee that calls to the listeners could be made concurrently as long as
those listeners were themselves thread safe. This commit adds a new class that
restores that behaviour for anybody who needs it.
