Improved new_window handling.

Remove new_window hook.
Plugin informed of all windows via events when first loaded.
init_qt no longer needed for exchange_rate plugin.

gui/qt/main_window.py

@@ -182,8 +182,6 @@ class ElectrumWindow(QMainWindow):
         self.fetch_alias()
         self.require_fee_update = False
         self.tx_notifications = []
-        # hook
-        run_hook('new_window', self)
 
     def is_hidden(self):
         return self.isMinimized() or self.isHidden()