Show ambient temperature and warp factor in the GUI's status bar

Adds a 'System' channel for static, global info that doesn't belong
to any one task - sent once at startup directly from brewpi.py's
__main__ (a client connecting later still gets it via the
dispatcher's global_state replay on subscribe, same mechanism
SudTask's startup Name/Description already relies on).

Moves warp_factor out of SudTask's startup message and into this new
channel, since it's not actually Sud-specific - it's now available
(and the status bar shows it) even without a Sud configured. The
forecast plot's progress-line scaling switches to the same general
Window.warp_factor field.

Displayed via a permanent status bar widget (not cleared by the
existing transient step/schedule showMessage() calls).

tasks/sud.py

@@ -24,7 +24,6 @@ class SudTask(ATask):
 		# its own, so hold_remaining must be ticked by dt explicitly to get
 		# the same speedup instead of running in real time.
 		self.dt = dt
-		self.warp_factor = dt / interval
 		self.msg_handler = msg_handler
 		msg_handler.set_recv_handler(self.recv)
 
@@ -112,11 +111,7 @@ class SudTask(ATask):
 		self.sud.set_on_changed('user_message', self.on_user_message_changed)
 		self.sud.set_on_changed('hold_remaining', ChangedFloat(self.on_hold_remaining_changed, prec=0).set)
 
-		asyncio.create_task(self.send({
-			'Name': self.sud.name,
-			'Description': self.sud.description,
-			'WarpFactor': self.warp_factor,
-		}))
+		asyncio.create_task(self.send({'Name': self.sud.name, 'Description': self.sud.description}))
 
 		while True:
 			if self.sud.state == SudState.RAMPING: