Routing dialog and MIDI receive toggle for plugins

More control effects plugins is needed.  For example, a VSTi instrument may ignore audio input and therefore discard a plugged in guitar input.  Using the audio "mix" parameter both the VSTi output and the guitar input are combined so the VSTi instrument can be played together with the guitar.

This series adds a MIDI receive parameter for effects plugins.  This makes it possible to prevent plugins from responding to unwanted MIDI messages.

This series also moves the audio "mix" and MIDI receive parameters into a dedicated effect plugin routing dialog.