Use two sockets for video and control

The socket used the device-to-computer direction to stream the video and
the computer-to-device direction to send control events.

Some features, like copy-paste from device to computer, require to send
non-video data from the device to the computer.

To make them possible, use two sockets:
 - one for streaming the video from the device to the client;
 - one for control/events in both directions.

app/src/server.h

@@ -11,7 +11,8 @@ struct server {
     char *serial;
     process_t process;
     socket_t server_socket; // only used if !tunnel_forward
-    socket_t device_socket;
+    socket_t video_socket;
+    socket_t control_socket;
     uint16_t local_port;
     bool tunnel_enabled;
     bool tunnel_forward; // use "adb forward" instead of "adb reverse"
@@ -22,7 +23,8 @@ struct server {
     .serial = NULL,                   \
     .process = PROCESS_NONE,          \
     .server_socket = INVALID_SOCKET,  \
-    .device_socket = INVALID_SOCKET,  \
+    .video_socket = INVALID_SOCKET,   \
+    .control_socket = INVALID_SOCKET, \
     .local_port = 0,                  \
     .tunnel_enabled = false,          \
     .tunnel_forward = false,          \