gui: add a persisted user config; remember the last Sud file dialog dir

New client/user_config.py - a small JSON-backed key/value store at
~/.config/brewpi/gui.json for GUI preferences that should survive
restarts (distinct from the server's config.json and from sude/*.json
schedules). First use: Save/Load Sud's file dialogs now open in
whichever directory was last used, instead of always starting at cwd.

client/brewpi_gui.py

@@ -5,6 +5,7 @@ import json
 import sys
 import threading
 import time
+from pathlib import Path
 import matplotlib
 matplotlib.use("Qt5Agg")
 from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg
@@ -15,6 +16,7 @@ from ws.client.ws_client import WsClient
 from ws.message import MessageDispatcherSync
 from queue import Queue
 import asyncio
+from user_config import UserConfig
 
 # str(SudState.X) values (as sent in the 'State' message) that count as
 # "running" for enabling/disabling the Start/Pause/Stop toolbar actions.
@@ -234,6 +236,7 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 		QtWidgets.QMainWindow.__init__(self)
 		self.setWindowIcon(QtGui.QIcon('res/beer.png'))
 		self.user_message_signal.connect(self.on_user_message_signal)
+		self.user_config = UserConfig()
 
 		loop = asyncio.new_event_loop()
 		asyncio.set_event_loop(loop)
@@ -569,16 +572,20 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 		# Pick the destination up front, on the GUI thread - the actual
 		# write happens in on_sud_changed(), which runs off a worker thread
 		# and must not touch Qt (e.g. open a file dialog) itself.
-		path, _ = QtWidgets.QFileDialog.getSaveFileName(self, "Save Sud", filter="JSON files (*.json)")
+		start_dir = self.user_config.get('sud_dir', '')
+		path, _ = QtWidgets.QFileDialog.getSaveFileName(self, "Save Sud", start_dir, filter="JSON files (*.json)")
 		if not path:
 			return
+		self.user_config.set('sud_dir', str(Path(path).parent))
 		self.sud_save_path = path
 		self.msg_sud.send({'Save': True})
 
 	def on_action_sud_load(self):
-		path, _ = QtWidgets.QFileDialog.getOpenFileName(self, "Load Sud", filter="JSON files (*.json)")
+		start_dir = self.user_config.get('sud_dir', '')
+		path, _ = QtWidgets.QFileDialog.getOpenFileName(self, "Load Sud", start_dir, filter="JSON files (*.json)")
 		if not path:
 			return
+		self.user_config.set('sud_dir', str(Path(path).parent))
 		with open(path) as f:
 			data = json.load(f)
 		self.msg_sud.send({'Load': data})

client/user_config.py

@@ -0,0 +1,27 @@
+import json
+from pathlib import Path
+
+DEFAULT_PATH = Path.home() / ".config" / "brewpi" / "gui.json"
+
+
+class UserConfig:
+	"""Small persisted key/value store for GUI preferences that should
+	survive across restarts (e.g. the last directory used for a file
+	dialog) - local user state, not version-controlled config."""
+
+	def __init__(self, path=DEFAULT_PATH):
+		self.path = path
+		self.data = {}
+		if self.path.exists():
+			try:
+				self.data = json.loads(self.path.read_text())
+			except (json.JSONDecodeError, OSError):
+				self.data = {}
+
+	def get(self, key, default=None):
+		return self.data.get(key, default)
+
+	def set(self, key, value):
+		self.data[key] = value
+		self.path.parent.mkdir(parents=True, exist_ok=True)
+		self.path.write_text(json.dumps(self.data, indent='\t'))