Rework TUI: single-threaded curses, scrollback buffer, server menu

- All curses drawing on the main thread; stdin polled together with the
  ZMQ sockets, Ctrl-C only sets a flag
- Output kept in a 2000-line buffer and redrawn from it, so resizes keep
  the history; PgUp/PgDn and mouse wheel scroll it
- Messages without trailing newline continue the current line (status
  table columns arrive as separate messages)
- Too-small terminal shows a warning instead of exiting
- Start without arguments shows a server selection menu
- Colors on the terminal default background; version 0.9.0

lib/network.py

@@ -89,10 +89,6 @@ class RconConnection:
         self.socket.send(command)
         logger.info(f'Sent command: {command}')
 
-    def poll(self, timeout):
-        """Poll for messages"""
-        return self.socket.poll(timeout)
-
     def recv_message(self):
         """Receive a message (non-blocking)"""
         try: