Improve ImportKeys popup closing/cancelling

- Once the importing process starts, only allow the popup to be closed
  by pressing pressing esc or clicking the Cancel button, to prevent
  accidental clicks

- When closing the popup without clicking the Cancel button, cancel the
  import process

src/gui/Dialogs/ImportKeys.qml

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 
 import QtQuick 2.12
+import QtQuick.Controls 2.12
 import Qt.labs.platform 1.1
 import "../Popups"
 import "../PythonBridge"
@@ -27,7 +28,7 @@ HFileDialogOpener {
             qsTr("Passphrase used to protect this file:")
         okText: qsTr("Import")
 
-        onCancelled: if (importFuture) importFuture.cancel()
+        onClosed: if (importFuture) importFuture.cancel()
 
 
         property url file: ""
@@ -73,5 +74,11 @@ HFileDialogOpener {
                 if (unknown) utils.showError(type, traceback, uuid)
             })
         }
+
+
+        Binding on closePolicy {
+            value: Popup.CloseOnEscape
+            when: importing
+        }
     }
 }