qt: move window.get{Open,Save}FileName to util

Sometimes we want its "remember path" behaviour but it does not make sense to
parent the dialog from main window. When so, caller code no longer needs to
get a reference to a main window.

Also rm last usages of get_parent_main_window().

electrum/gui/qt/seed_dialog.py

@@ -105,7 +105,7 @@ class SeedLayout(QVBoxLayout):
             if for_seed_words:
                 self.seed_e = ButtonsTextEdit()
             else:  # e.g. xpub
-                self.seed_e = ShowQRTextEdit()
+                self.seed_e = ShowQRTextEdit(config=self.config)
             self.seed_e.setReadOnly(True)
             self.seed_e.setText(seed)
         else:  # we expect user to enter text