'hook up ChannelConfig GUI to NJClient (issue #25)'

* now allows zero local channels for creating stereo pair if only two inputs
* added two SortedSets holding available input channels for ChannelConfig GUI
* added IsInitialized flag for destroying corrupted storage during init phase
* removed LinJam::GetOrAddRemoteUser() and LinJam::GetOrAddRemoteChannel() helpers
* added CreateLocalChannel() , InstantiateLocalChannel() , TrimStereoChannelName() ,
        GetNumActiveChannels() , GetNumVacantChannels() , GetChannelDisplayName helpers
* factored channel group resizing out of Mixer and into Channels
* minor adjustments to mixer layout ready for alpha fading hidden remotes