Don't freeze the GUI thread on disconnect

ws_client.disconnect() blocks until the close handshake completes
(observed ~10s when the server doesn't ack promptly), and was being
called straight from the Connect/Disconnect button's click handler -
joining that wait on the GUI thread, so the click appeared to do
nothing until the timeout passed. Run it off a throwaway thread
instead; on_ws_connect_changed() already flips the button once the
disconnect actually finishes.

client/brewpi_gui.py

@@ -3,6 +3,7 @@ from main_window import Ui_MainWindow
 from PyQt5 import QtWidgets, QtGui, QtCore
 import json
 import sys
+import threading
 import time
 import matplotlib
 matplotlib.use("Qt5Agg")
@@ -246,7 +247,12 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 			self.plot_power_set, self.plot_power_eff)
 
 	def disconnect(self):
-		self.ws_client.disconnect()
+		# ws_client.disconnect() blocks until the websocket's close handshake
+		# completes (or times out, ~10s if the server doesn't ack promptly),
+		# joining the connection's background thread - run it off a throwaway
+		# thread so that wait doesn't freeze the GUI. on_ws_connect_changed()
+		# still flips the button once the disconnect actually finishes.
+		threading.Thread(target=self.ws_client.disconnect, daemon=True).start()
 
 	def on_btn_connect_clicked(self):
 		if self.connected: