When soled one FX channel, unmute send and receive channels, to allow complex FX channel routing (BUS, SENDs, etc.)

Activate also SEND channels recursively

…eForSoloRecursive

Conflicts:
* include/Mixer.h

Make the mixer channel index `m_channelIndex` private and add getters and
setters. Also add a method to determine if the channel is the master
channel.

Move `m_channelIndex` to the end of the initialization list of the
`MixerChannel` constructor because it is now the last member in the
header.

Adjust all clients to make use of the new methods.