Previously, IEditController parameter indices were being used to index
into the AudioProcessor parameter array, but these parameter indices are
not guaranteed to point to the same parameter (parameter groups may
cause reordering on JUCE's side). Now, we use the IEditController
indices universally.

Audition incorrectly increments the ref-count of the IPlugView* returned from IEditController::createView() and never fully releases it. This commit reverts e0306d25 for Audition to ensure that an editor is always returned, otherwise Audition displays a blank GUI.

MIDI FX AudioUnits can retrieve their current sample rate from an output
bus. However, it's reasonable for a JUCE MIDI FX plugin to require 0
output buses, in which case the correct sample rate cannot be retrieved.

This patch modifies the AU wrapper to ensure that MIDI FX plugins will
always have at least one output bus. This bus does not necessarily
correspond to a matching bus in the AudioProcessor, and it will not be
passed to AudioProcessor::processBlock. With the output bus in place,
the correct sample rate is reported in `prepareToPlay`.

Previously, all VST3 parameter changes were signalled to the host via
performEdit, regardless of the originating thread. This breaks the
contract of the IEditController interface, which requires that all calls
are made from the UI thread.

This change modifies the VST3 wrapper so that it will only call
performEdit when a parameter is set on the message thread. If a
parameter is set on the audio thread instead, then the parameter change
will be signalled to the host using
`ProcessData::outputParameterChanges`.

If your VST3 plugin uses a background thread to update parameters for
some reason, you'll encounter data races. Please don't do that.

When loading a JUCE plugin in Live, Live was producing diagnostic
messages in its Log.txt warning that restartComponent was being called
from a non-UI thread. This change should silence this warning, and
improve compliance with the VST3 spec.

Previously, the AudioProcessorPlayer would always match the
AudioProcessor's bus configuration to the requested bus configuration,
even if the processor did not explicitly support the requested
configuration.

Now, if the requested configuration has one or fewer input channels, the
AudioProcessorPlayer will attempt to find a multi-input channel layout
for which `checkBusesLayoutSupported` returns true, and will use such a
layout if it exists. Otherwise, as a last resort, it will fall back to
using the channel layout requested by the AudioProcessorPlayer.

If the AudioProcessorPlayer has no input channels, but the wrapped
processor is initialised with multiple input channels, each of these
inputs will be fed with silence.

If the AudioProcessorPlayer has a single input channel, but the wrapped
processor is initialised with multiple input channels, each input
channel will be fed with a copy of the AudioProcessorPlayer's mono
input.

This patch also moves PluginHostType into juce_audio_processors in order
to allow hiding of function implementations.

Reports the correct host name for Apple hosts on M1 which host plugins
in a separate process.

Affected hosts are:
- Logic Pro
- GarageBand
- MainStage
- Final Cut Pro