GUI: add a spinbox to pre-dial the simulated Pot's reset temperature

btn_pot_reset always reset the simulated Pot to the ambient
temperature. Add doubleSpinBox_pot_temp next to it (shown/hidden
together, same as the button) so the user can dial in a different
starting temperature before committing it - it seeds from the same
remembered ambient value on startup but is otherwise independent and
not persisted.

tasks/pot.py's PotTask.recv() now resets the plant to whatever
temperature the 'Reset' message carries, falling back to ambient for
the bare True a caller might still send.

client/brewpi_gui.py

@@ -375,10 +375,19 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 			self.doubleSpinBox_ambient.setValue(remembered_ambient)
 			self.doubleSpinBox_ambient.blockSignals(False)
 		self.doubleSpinBox_ambient.valueChanged.connect(self.on_ambient_entry_changed)
+		# Pre-dial the Pot reset target at the same starting point btn_pot_
+		# reset used to hardcode (ambient) - purely a one-off starting value
+		# for the user to optionally dial away from before clicking the
+		# button, so unlike the ambient spinbox it's never persisted/synced
+		# again afterwards.
+		if remembered_ambient is not None:
+			self.doubleSpinBox_pot_temp.setValue(remembered_ambient)
 		self.btn_pot_reset.clicked.connect(self.on_action_pot_reset)
 		# Only meaningful (and only shown) once the server confirms its Pot
 		# is simulated - see on_system_changed()'s "PlantSim" handling.
 		self.btn_pot_reset.setVisible(False)
+		self.label_pot_temp.setVisible(False)
+		self.doubleSpinBox_pot_temp.setVisible(False)
 
 		self.send_data = Queue()
 		self.slider_pwr_initial_update = True
@@ -509,6 +518,8 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 			# showing whatever the user last set rather than blanking it.
 			self.ambient_temp = None
 			self.btn_pot_reset.setVisible(False)
+			self.label_pot_temp.setVisible(False)
+			self.doubleSpinBox_pot_temp.setVisible(False)
 			self.warp_factor = 1.0
 			self.sud_elapsed_seconds = None
 			self.sud_name = ''
@@ -554,10 +565,12 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 				self.warp_factor = msg['WarpFactor']
 			elif "PlantSim" in key:
 				self.btn_pot_reset.setVisible(msg['PlantSim'])
+				self.label_pot_temp.setVisible(msg['PlantSim'])
+				self.doubleSpinBox_pot_temp.setVisible(msg['PlantSim'])
 		self.update_status_env_label()
 
 	def on_action_pot_reset(self):
-		self.msg_pot.send({'Reset': True})
+		self.msg_pot.send({'Reset': self.doubleSpinBox_pot_temp.value()})
 
 	def on_ambient_entry_changed(self, value):
 		# Persisted on shutdown instead (see closeEvent()), not on every