Split channels model into separated file

Firstly channels model was joined with general model because they are
placed on the same General tab. But it caused some problems:

1. Channels model will be used by patterns model, that will be located
in a different tab.
2. *ChannelsDelegate* file looks orphaned in files list, there is no
sense on first glance, where this class is used. If *ChannelsModel* will
be located near *ChannelsDelegate*, it will be oblivious, that the
second thing is used by the first one.