[mythtv] Coexistence of distinct channel types.

[Ramon Roca]

Being thinking on that for a while, and having as a goal a seamless integration 
of all channel types with a single preset and supporting coexistence between 
them, i do see this way for supporting it:

Channel table, is the base table for channels, if channel type is analog, self 
contains all the information. Has also the unique preset. If flags to another 
channel type, has the fk (might be even chanid) to join with the appropiate 
tables.

Channel_dvb (and others that might follow in the future, like lists, hdtv, 
etc), provides specific info of the channel.

Using current channel table as a base table, is a simple way to consolidate 
everything with a single key and preset, assures transparency to current users 
etc. Also simplifies dev, since currently mythtv is centered on this table.