This naming is a bit hairy. I chose CTextMessage because this type of
message carries a text argument. Alternatively, it could be also called
CPrivateMessage, because the IRC protocol calls it PRIVMSG. On the other
hand, ZNC module hooks use the "Priv" naming convention for private
messages. It would look a bit weird to have OnChanMsg(CPrivMessage)...
More details and reasoning of the merge in the previous commit message.
OnUserAction(), OnUserCTCP(), and OnUserNotice() don't separate private
private and channel messages. A module could even redirect a message by
modifying its target, so technically, if they were two distinct types,
the type of a message could change on the way.
The original reason for the separation was that at some point during
CMessage development, GetChan() didn't exist in CPrivXxx, but only in
CChanXxx message types. To achieve cleaner implementation, the getter
was later promoted to CMessage and made return nullptr for non-channel
specific messages. From this point of view, the separation is also no
longer necessary since the CPrivXxx and CChanXxx types are API-wise
identical with each other.
