client API: introduce numeric log levels

Maybe using strings for log levels was a mistake (too broad and too
impractical), so I'm adding numeric log level at least for the receiver
side. This makes it easier to map mpv log levels to other logging
systems.

I'm still too stingy to add a function to set the log level by a numeric
value, though.

The numeric values are not directly mapped to the internal mpv values,
because then almost every file in mpv would have to include the client
API header.

Coalesce this into API version 1.6, since 1.6 was bumped just yesterday.