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/coldcard/qt.py

@@ -5,7 +5,8 @@ import copy
 from PyQt5.QtCore import Qt, pyqtSignal
 from PyQt5.QtWidgets import QPushButton, QLabel, QVBoxLayout, QWidget, QGridLayout
 
-from electrum.gui.qt.util import WindowModalDialog, CloseButton, Buttons
+from electrum.gui.qt.util import (WindowModalDialog, CloseButton, Buttons, getOpenFileName,
+                                  getSaveFileName)
 from electrum.gui.qt.transaction_dialog import TxDialog
 from electrum.gui.qt.main_window import ElectrumWindow
 
@@ -64,8 +65,13 @@ class Plugin(ColdcardPlugin, QtPluginBase):
 
         basename = wallet.basename().rsplit('.', 1)[0]        # trim .json
         name = f'{basename}-cc-export.txt'.replace(' ', '-')
-        fileName = main_window.getSaveFileName(_("Select where to save the setup file"),
-                                                        name, "*.txt")
+        fileName = getSaveFileName(
+            parent=main_window,
+            title=_("Select where to save the setup file"),
+            filename=name,
+            filter="*.txt",
+            config=self.config,
+        )
         if fileName:
             with open(fileName, "wt") as f:
                 ColdcardPlugin.export_ms_wallet(wallet, f, basename)
@@ -191,10 +197,14 @@ class CKCCSettingsDialog(WindowModalDialog):
 
     def start_upgrade(self, client):
         # ask for a filename (must have already downloaded it)
-        mw = self.window
         dev = client.dev
 
-        fileName = mw.getOpenFileName("Select upgraded firmware file", "*.dfu")
+        fileName = getOpenFileName(
+            parent=self,
+            title="Select upgraded firmware file",
+            filter="*.dfu",
+            config=self.window.config,
+        )
         if not fileName:
             return
 
@@ -220,7 +230,7 @@ class CKCCSettingsDialog(WindowModalDialog):
             if magic != FW_HEADER_MAGIC:
                 raise ValueError("Bad magic")
         except Exception as exc:
-            mw.show_error("Does not appear to be a Coldcard firmware file.\n\n%s" % exc)
+            self.window.show_error("Does not appear to be a Coldcard firmware file.\n\n%s" % exc)
             return
 
         # TODO: 
@@ -228,7 +238,7 @@ class CKCCSettingsDialog(WindowModalDialog):
         # - warn them about the reboot?
         # - length checks
         # - add progress local bar
-        mw.show_message("Ready to Upgrade.\n\nBe patient. Unit will reboot itself when complete.")
+        self.window.show_message("Ready to Upgrade.\n\nBe patient. Unit will reboot itself when complete.")
 
         def doit():
             dlen, _ = dev.upload_file(firmware, verify=True)

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
 

electrum/plugins/trezor/qt.py

@@ -8,7 +8,7 @@ from PyQt5.QtWidgets import (QVBoxLayout, QLabel, QGridLayout, QPushButton,
                              QMessageBox, QFileDialog, QSlider, QTabWidget)
 
 from electrum.gui.qt.util import (WindowModalDialog, WWLabel, Buttons, CancelButton,
-                                  OkButton, CloseButton, PasswordLineEdit)
+                                  OkButton, CloseButton, PasswordLineEdit, getOpenFileName)
 from electrum.i18n import _
 from electrum.plugin import hook
 from electrum.util import bh2u
@@ -542,8 +542,11 @@ 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