kivy: use exchange rates in amount dialog

plugins/exchange_rate/kivy.py

@@ -1,3 +1,11 @@
 from exchange_rate import FxPlugin
+from electrum.plugins import hook
+
 class Plugin(FxPlugin):
-    pass
+    @hook
+    def load_wallet(self, wallet, window):
+        self.window = window
+
+    def on_quotes(self):
+        self.print_error("on quotes", self.ccy)
+        self.window.fiat_unit = self.ccy