This allows real-time threads to join an audio workgroup on Apple platforms.

This patch also updates the MidiDemo to automatically refresh the device
lists when the set of available devices changes.

Changed the lifetime of the default MIDI oputput.
Removed the need for Timer workaround in AudioDeviceSelectorComponent.

This fixes an issue where specifying an audio device type before calling
initialise could cause a different device to become active, even if the
requested device type had usable devices.

With this change in place, a new device type will only be selected if
the current device type has no devices.

This fixes warnings that are emitted when building with the `-analyze`
flag enabled.