qml: add config setting to allow screenshots

On Android, we disallow screenshots on screens where the seed is visible.
(The seed is extremely sensitive data that should not be stored digitally without
significant precautions but it's also cumbersome to write down or memorise, so
some people instinctively just try to take a screenshot of it when creating a wallet.)
We do this by using the built-in OS mechanism of setting FLAG_SECURE on the window.
However, on some devices with custom ROMs (one report from LineageOS, one from /e/OS),
unsetting FLAG_SECURE crashes the application for some reason.

As a workaround, this commit adds a config setting into the Preferences,
to disable this mechanism and just always allow screenshots.
(note that you can get into the qml Preferences before creating/opening any wallet)

ref https://github.com/spesmilo/electrum/issues/8522

electrum/gui/qml/components/Preferences.qml

@@ -368,6 +368,24 @@ Pane {
                             wrapMode: Text.Wrap
                         }
                     }
+
+                    RowLayout {
+                        Layout.columnSpan: 2
+                        Layout.fillWidth: true
+                        spacing: 0
+                        Switch {
+                            id: alwaysAllowScreenshots
+                            onCheckedChanged: {
+                                if (activeFocus)
+                                    Config.alwaysAllowScreenshots = checked
+                            }
+                        }
+                        Label {
+                            Layout.fillWidth: true
+                            text: qsTr('Always allow screenshots')
+                            wrapMode: Text.Wrap
+                        }
+                    }
                 }
 
             }
@@ -392,6 +410,7 @@ Pane {
         useTrampolineRouting.checked = !Config.useGossip
         useFallbackAddress.checked = Config.useFallbackAddress
         enableDebugLogs.checked = Config.enableDebugLogs
+        alwaysAllowScreenshots.checked = Config.alwaysAllowScreenshots
         useRecoverableChannels.checked = Config.useRecoverableChannels
         syncLabels.checked = AppController.isPluginEnabled('labels')
     }

electrum/gui/qml/qeapp.py

@@ -327,6 +327,8 @@ class QEAppController(BaseCrashReporter, QObject):
     def secureWindow(self, secure):
         if not self.isAndroid():
             return
+        if self.config.GUI_QML_ALWAYS_ALLOW_SCREENSHOTS:
+            return
         if self._secureWindow != secure:
             jpythonActivity.setSecureWindow(secure)
             self._secureWindow = secure

electrum/gui/qml/qeconfig.py

@@ -169,6 +169,16 @@ class QEConfig(AuthMixin, QObject):
         self.config.GUI_ENABLE_DEBUG_LOGS = enable
         self.enableDebugLogsChanged.emit()
 
+    alwaysAllowScreenshotsChanged = pyqtSignal()
+    @pyqtProperty(bool, notify=alwaysAllowScreenshotsChanged)
+    def alwaysAllowScreenshots(self):
+        return self.config.GUI_QML_ALWAYS_ALLOW_SCREENSHOTS
+
+    @alwaysAllowScreenshots.setter
+    def alwaysAllowScreenshots(self, enable):
+        self.config.GUI_QML_ALWAYS_ALLOW_SCREENSHOTS = enable
+        self.alwaysAllowScreenshotsChanged.emit()
+
     useRecoverableChannelsChanged = pyqtSignal()
     @pyqtProperty(bool, notify=useRecoverableChannelsChanged)
     def useRecoverableChannels(self):

electrum/simple_config.py

@@ -1105,6 +1105,7 @@ This will result in longer routes; it might increase your fees and decrease the
     GUI_QML_USER_KNOWS_PRESS_AND_HOLD = ConfigVar('user_knows_press_and_hold', default=False, type_=bool)
     GUI_QML_ADDRESS_LIST_SHOW_TYPE = ConfigVar('address_list_show_type', default=1, type_=int)
     GUI_QML_ADDRESS_LIST_SHOW_USED = ConfigVar('address_list_show_used', default=False, type_=bool)
+    GUI_QML_ALWAYS_ALLOW_SCREENSHOTS = ConfigVar('android_always_allow_screenshots', default=False, type_=bool)
 
     BTC_AMOUNTS_DECIMAL_POINT = ConfigVar('decimal_point', default=DECIMAL_POINT_DEFAULT, type_=int)
     BTC_AMOUNTS_FORCE_NZEROS_AFTER_DECIMAL_POINT = ConfigVar(