Autohost interface: add "label" event

https://github.com/beyond-all-reason/spring/blob/BAR105/rts/Net/AutohostInterface.cpp

There is a "player chat" event (id 13), but it doesn't handle labels, which can also contain text and are often used like chat. The request is to add a "player added label with text" event.

Some remarks:
* my use case only cares about labels with text, but empty labels should probably produce an event as well (no reason not to).
* labels are a specific form of scribblings (line drawings) but I am not requesting scribblings to produce an event.

Start in rts/Net/AutohostInterface.cpp, linked from the issue, and inspect the existing player chat event with id 13 alongside label handling. Trace how labels are represented and emitted, then verify that text and empty labels produce a player-label event while scribblings do not. Done means the autohost interface exposes the requested label event consistently with the existing chat event.

Written by the indexing model from the issue text.