Make fullscreen images focused when they're opened

Otherwise keyboard shortcuts don't work until you tap the image, which
makes no sense.

BUG: 484322

src/qml/QrCodeMaximizeComponent.qml

@@ -14,6 +14,8 @@ Components.AbstractMaximizeComponent {
     property color avatarColor
     required property string avatarSource
 
+    onOpened: forceActiveFocus()
+
     Shortcut {
         sequences: [StandardKey.Cancel]
         onActivated: root.close()