Add EffectPlugin->receiveMidi property

Add property to enable/disable MIDI input.  This is useful because some
plugins respond to all MIDI messages and have no UI to let the user
disable this.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Add "receive MIDI" checkbox to EffectSettingsPage

Allow the user to disable MIDI input on plugins.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Honor EffectPlugin->receiveMidi in EffectProcessor

Don't send MIDI events to plugins that have receive MIDI disabled.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Add effect plugin audio/MIDI routing dialog

Move the audio wet/dry mix and receive MIDI parameters into its own
dialog.  In the future we may also add audio channel mapping (Left/Mono,
Right) for plugins with more than 2 audio in/out channels to the routing
dialog.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>