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/qrcodewidget.py

@@ -9,8 +9,9 @@ from PyQt5.QtWidgets import (
 )
 
 from electrum.i18n import _
+from electrum.simple_config import SimpleConfig
 
-from .util import WindowModalDialog, get_parent_main_window, WWLabel
+from .util import WindowModalDialog, WWLabel, getSaveFileName
 
 
 class QRCodeWidget(QWidget):
@@ -95,15 +96,17 @@ class QRDialog(WindowModalDialog):
 
     def __init__(
             self,
+            *,
             data,
             parent=None,
             title="",
             show_text=False,
-            *,
             help_text=None,
             show_copy_text_btn=False,
+            config: SimpleConfig,
     ):
         WindowModalDialog.__init__(self, parent, title)
+        self.config = config
 
         vbox = QVBoxLayout()
 
@@ -122,11 +125,12 @@ class QRDialog(WindowModalDialog):
         hbox.addStretch(1)
 
         def print_qr():
-            main_window = get_parent_main_window(self)
-            if main_window:
-                filename = main_window.getSaveFileName(_("Select where to save file"), "qrcode.png")
-            else:
-                filename, __ = QFileDialog.getSaveFileName(self, _("Select where to save file"), "qrcode.png")
+            filename = getSaveFileName(
+                parent=self,
+                title=_("Select where to save file"),
+                filename="qrcode.png",
+                config=self.config,
+            )
             if not filename:
                 return
             p = qrw.grab()