msg: set an explicit overflow message

So client API users don't have to worry about this specifically.

Also document the overflow case. (Not sure if we really need to do
this; maybe it'd be better not to, since this just adds more noise
to the docs.)
msg, client API: never send the status line as log message

The status line is a bit special; for example it uses special control
codes by design and is not terminated with a newline character in order
to update it on the terminal without scrolling. It's not helpful for
client API users either, and would require special-casing them
(emulating aspects of a terminal?). Also, the status line code is
explicitly disabled on osd.c unless the --terminal option is enabled,
so there was no good way to even enable the status line for the API.

Just pretend that the status line does not exist as far as the client
API is concerned. It won't be sent as MPV_EVENT_LOG_MESSAGE.
msg, client API: buffer partial lines

The API could return partial lines, meaning the message could stop
in the middle of a line, and the next message would have the rest of
it (or just the next part of it). This was a pain for the user, so do
the nasty task of buffering the lines ourselves.

Now only complete lines are sent. To make things even easier for the
API user, don't put multiple lines into a single event, but split them.

The terminal output code needed something similar (inserting a prefix
header on start of each line). To avoid code duplication, this commit
refactors the terminal output so that lines are split in a single
place.
DOCS/client_api_examples: qtexample: add a log window

For the purpose of demonstration.

Also make the windows larger. I'm not exactly sure how Qt determines
the default window sizes, but here they are a bit tiny, so force them
larger.
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.