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/installwizard.py

@@ -15,7 +15,6 @@ from electrum import util
 import seed_dialog
 from network_dialog import NetworkDialog
 from util import *
-from amountedit import AmountEdit
 
 from electrum.plugins import always_hook, run_hook
 from electrum.mnemonic import prepare_seed