Added notifications for endpoint additions/removals, and moved destinationRemoved/sourceRemoved delegate invocations to after source/destination has been removed
Implemented source delegate multiplexing: Can now have multiple delegates per source; don't retain PGMidi delegate (avoid retain cycles)
Replaced enableNetwork: with readwrite networkEnabled property
Added +midiAvailable check
Fixed endpoint name retrieval
Updated default MIDI client identifiers to "PGMidi" from "MidiMonitor"
Implemented support for Virtual MIDI
Added regular device scanning to overcome apparent unreliabilities in Core MIDI's notification system