Updates for exchange_rate plugin

Have AmountEdit return an int if is_int, otherwise a Decimal
Set the tray tooltip unconditionally.
More verbose logging for exchage_rate plugin.
Get rate_float from Coindesk as rate can have commas.
Plugin tracks windows itself, and doesn't create its own
members in the window objects.
Clean up the edit handling.

gui/qt/amountedit.py

@@ -63,10 +63,9 @@ class AmountEdit(MyLineEdit):
 
     def get_amount(self):
         try:
-            x = int(str(self.text()))
+            return (int if self.is_int else Decimal)(str(self.text()))
         except:
             return None
-        return x
 
 
 class BTCAmountEdit(AmountEdit):