Convert MIDIManager to global singleton

[orchetect]

Proposal

There are numerous arguments in favor of making MIDIManager an implicit global singleton rather than something that has to be instanced.

The core benefits:

• manager does not need to be stored in a variable by the consumer
• manager is available globally and does not have to be injected into methods that require a reference to it
• manager instance would no longer need to be propagated through SwiftUI environment

The conceivable use cases of creating more than one manager instance is almost zero. The only area where it becomes useful is in unit testing, where it could be possible to retain the ability to instance multiple managers internally. (For example, tests involving sending events to/from legacy Core MIDI API to a receiver using new API which requires two managers.)

Current Implementation

// manager instance which must be stored in a variable, typically at app-leveo scope
let midiManager = MIDIManager(
    clientName: "TestAppMIDIManager",
    model: "TestApp",
    manufacturer: "MyCompany"
)

try MIDIManager.start()

Suggested Implementation

// MIDIManager.shared will be a global static singleton instance
// the consumer must still set up the required parameters first before starting the manager,
// but instead of passing them as parameters to an initializer, a method on the singleton may be called instead:
MIDIManager.shared.setup(
    clientName: "TestAppMIDIManager",
    model: "TestApp",
    manufacturer: "MyCompany"
)

try MIDIManager.shared.start()

Or the two could be combined:

try MIDIManager.shared.start(
    clientName: "TestAppMIDIManager",
    model: "TestApp",
    manufacturer: "MyCompany"
)

Considerations

• Currently there are two manager classes: MIDIManager and ObservableMIDIManager. These may need to be merged or the former deprecated.
• It would need to be investigated how/if ObservableMIDIManager can be used as an observable object in SwiftUI views while it is a global singleton, without it being installed in an App or View struct.