Set monitor to L/R by default in AUv3 mode

[izzyreal]

I'm not 100% sure yet, but this might make sense, since the only way to sample when using the AUv3 is when using it as an AU effect, which often implies inserting VMPC2000XL on the channel that is producing the sound that is to be sampled. In that scenario, without monitoring (or audio pass-through) the user has no way to hear the audio produced by the audio chain up to VMPC2000XL.

On the other hand one could argue that it's better to set up a mix bus dedicated to VMPC2000XL. This way, the user won't have to drag a VMPC2000XL instance from one slot to another. One can choose to sample from a source by routing it to the bus. The caveat here is the additional channel required to also send the bus to the main output.

Enabling monitoring by default would hurt the dedicated-mix-bus scenario, because if the user has set up the additional channel, they will now hear the sampled sound source twice.