Have fields update as exchange rates do

Currently the exchange rates plugin shows the converted
rate at the time of last user input.  If the fx rate
changes the send and receive tabs do not update.

This makes them update.  It also means that when enabling
the plugin, if the user had input a BTC amount in the send
or receive, the other fields will refresh.  This didn't
used to happen - they stayed blank.

plugins/exchange_rate.py

@@ -95,6 +95,7 @@ class Exchanger(threading.Thread):
         with self.lock:
             self.quote_currencies = rates
             self.parent.set_currencies(rates)
+        self.parent.refresh_fields()
 
     def run(self):
         self.is_running = True
@@ -190,6 +191,7 @@ class Plugin(BasePlugin):
         self.exchanger.start()
         self.win = None
         self.resp_hist = {}
+        self.fields = {}
 
     @hook
     def init_qt(self, gui):
@@ -496,6 +498,11 @@ class Plugin(BasePlugin):
     def fiat_unit(self):
         return self.config.get("currency", "EUR")
 
+    def refresh_fields(self):
+        '''Update the display at the new rate'''
+        for field in self.fields.values():
+            field.textEdited.emit(field.text())
+
     def add_send_edit(self):
         self.send_fiat_e = AmountEdit(self.fiat_unit)
         btc_e = self.win.amount_e
@@ -513,6 +520,7 @@ class Plugin(BasePlugin):
     def connect_fields(self, btc_e, fiat_e, fee_e):
         def fiat_changed():
             fiat_e.setStyleSheet(BLACK_FG)
+            self.fields[(fiat_e, btc_e)] = fiat_e
             try:
                 fiat_amount = Decimal(str(fiat_e.text()))
             except:
@@ -528,6 +536,7 @@ class Plugin(BasePlugin):
         fiat_e.textEdited.connect(fiat_changed)
         def btc_changed():
             btc_e.setStyleSheet(BLACK_FG)
+            self.fields[(fiat_e, btc_e)] = btc_e
             if self.exchanger is None:
                 return
             btc_amount = btc_e.get_amount()
@@ -541,6 +550,7 @@ class Plugin(BasePlugin):
                 fiat_e.setCursorPosition(pos)
                 fiat_e.setStyleSheet(BLUE_FG)
         btc_e.textEdited.connect(btc_changed)
+        self.fields[(fiat_e, btc_e)] = btc_e
 
     @hook
     def do_clear(self):