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/qrtextedit.py

@@ -4,14 +4,15 @@ from electrum.i18n import _
 from electrum.plugin import run_hook
 from electrum.simple_config import SimpleConfig
 
-from .util import ButtonsTextEdit, MessageBoxMixin, ColorScheme
+from .util import ButtonsTextEdit, MessageBoxMixin, ColorScheme, getOpenFileName
 
 
 class ShowQRTextEdit(ButtonsTextEdit):
 
-    def __init__(self, text=None):
+    def __init__(self, text=None, *, config: SimpleConfig):
         ButtonsTextEdit.__init__(self, text)
-        self.setReadOnly(1)
+        self.config = config
+        self.setReadOnly(True)
         icon = "qrcode_white.png" if ColorScheme.dark_scheme else "qrcode.png"
         self.addButton(icon, self.qr_show, _("Show as QR code"))
 
@@ -23,7 +24,11 @@ class ShowQRTextEdit(ButtonsTextEdit):
             s = str(self.toPlainText())
         except:
             s = self.toPlainText()
-        QRDialog(s, parent=self).exec_()
+        QRDialog(
+            data=s,
+            parent=self,
+            config=self.config,
+        ).exec_()
 
     def contextMenuEvent(self, e):
         m = self.createStandardContextMenu()
@@ -44,7 +49,11 @@ class ScanQRTextEdit(ButtonsTextEdit, MessageBoxMixin):
         run_hook('scan_text_edit', self)
 
     def file_input(self):
-        fileName, __ = QFileDialog.getOpenFileName(self, 'select file')
+        fileName = getOpenFileName(
+            parent=self,
+            title='select file',
+            config=self.config,
+        )
         if not fileName:
             return
         try: