Log: Introduce the PRINT level and use it for console specific prints

This is a first try and is a bit ugly. The PRINT level is like
NOTICE except that a console can ask to be the sole receiver for
PRINTs (so that most commands output will stay on that console only)
So this commit:
 * adds a PRINT level and add the level to Log::Event
 * adds an entry point to Log to change the target for PRINTs
(no target specified = all console)
 * implement that console lock for commands and completion in consoles

The problem is that Com_Printf uses Cmd::GetEnv()->Print() which will
use Print() so commands like /map may end up putting everything in a
single console even though that's not what we want (some things should)
use NOTICE.