The CoreAudioKit (and on macOS, AudioUnit) frameworks are required to
host AudioUnit plugins. Hosts (especially those which don't use the
`juce_audio_utils` module) should use the new `PLUGINHOST_AU` parameter
to `juce_add_*` in order to add the correct preprocessor definition and
link the necessary frameworks.

This change adds the examples from `examples/Plugins` to the
AudioPluginHost, surfacing them as 'internal' plugins in the popup menu.

We also build this into the AudioPluginHost, as a convenient way to
check the midi outputs of plugins and devices.