MIDI only plugins are no longer provided valid audio buffers
Graph nodes are only executed when they have active connections

This change also allows the plugin to receive discrete layouts in hosts
that support them, while also maintaining support for
AudioChannelLayoutTags in Logic.

The comment at the top of reaper_plugin_fx_embed.h specifies that the
'index' parameter for the embedded UI feature will always be set to
effEditDraw.

This prevents test failures in the Steinberg VST3 validator.

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.