qt wizard: tweak GoBack behaviour to recalc inputs to previous dialog

follow-up f13f46c555

When on dialog n user presses "Back",
- previously, we went back to when dialog n-1 appeared
- now, go back to just after dialog n-2 finishes
This way, any calculations between when dialog n-2 finishes and
dialog n-1 appears will rerun, potentially populating dialog n-1 differently.

Namely if the user presses back on the confirm_seed_dialog, we want to
go back to the show_seed_dialog but with a freshly generated seed.

electrum/base_wizard.py

@@ -60,6 +60,9 @@ class ScriptTypeNotSupported(Exception): pass
 class GoBack(Exception): pass
 
 
+class ReRunDialog(Exception): pass
+
+
 class ChooseHwDeviceAgain(Exception): pass