enums: add quality_indicator messages

These messages are used to exchange Streaming Quality messages between
SPICE client and SPICE server:

- `SPICE_MSGC_MAIN_QUALITY_INDICATOR`: a client->server message on the
  Main channel. This message is used by the client to share streaming
  quality observation with the server and the adaptive streaming
  control center. The format of the string message is not specified
  yet, it will depend of the sender/receiver final implementation.

- `SPICE_MSG_DISPLAY_QUALITY_INDICATOR`: a server->client message on
  the Display channel. This message is used to inform the client about
  adaptive-streaming specific details. The client should use it to
  configure its quality-message generation algorithms. The format of
  the string message is not specified yet, it will depend of the
  sender/receiver final implementation.

As an example, a `SPICE_MSG_DISPLAY_QUALITY_INDICATOR` message can
inform the client about the requested framerate (say, 60 FPS); and the
client will send a `SPICE_MSGC_MAIN_QUALITY_INDICATOR` warning message
when the measured framerate drops below 45 FPS.

Signed-off-by: Kevin Pouget <kpouget@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>

spice/enums.h

@@ -507,6 +507,7 @@ enum {
     SPICE_MSG_DISPLAY_STREAM_ACTIVATE_REPORT,
     SPICE_MSG_DISPLAY_GL_SCANOUT_UNIX,
     SPICE_MSG_DISPLAY_GL_DRAW,
+    SPICE_MSG_DISPLAY_QUALITY_INDICATOR,
 
     SPICE_MSG_END_DISPLAY
 };