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/plugins/safe_t/qt.py

@@ -9,7 +9,7 @@ from PyQt5.QtWidgets import (QVBoxLayout, QLabel, QGridLayout, QPushButton,
                              QMessageBox, QFileDialog, QSlider, QTabWidget)
 
 from electrum.gui.qt.util import (WindowModalDialog, WWLabel, Buttons, CancelButton,
-                                  OkButton, CloseButton)
+                                  OkButton, CloseButton, getOpenFileName)
 from electrum.i18n import _
 from electrum.plugin import hook
 from electrum.util import bh2u
@@ -276,8 +276,11 @@ class SettingsDialog(WindowModalDialog):
             invoke_client('toggle_passphrase', unpair_after=currently_enabled)
 
         def change_homescreen():
-            dialog = QFileDialog(self, _("Choose Homescreen"))
-            filename, __ = dialog.getOpenFileName()
+            filename = getOpenFileName(
+                parent=self,
+                title=_("Choose Homescreen"),
+                config=config,
+            )
             if not filename:
                 return  # user cancelled