Allow the user to freeze channels on frozen instrument tracks

When the user clicks Freeze Instrument Channel, they are presented with the option to freeze either just the instrument, or the instrument as well as the channel.

As a workflow, it’s common for the user to freeze the instrument only while mixing, then to want to freeze channels as well as they make more committal mix decisions.

Unfortunately, when the user decides this, they cannot migrate from the first frozen state to the second frozen state.

Therefore the user must unfreeze the instrument, which loads the VSTi again, then freeze with the second option once again manually.

This is massively wasteful and tedious, and should be fixed.