iOS: Teach edit menu popover to have a readonly mode

We should have foreseen the need for customizing this menu in 2014 when
Controls 1 needed it, but here we are: Qt 5 is "done", this still wasn't
done, and it's too late for new enum values in 5.15. So the workaround is
to add a property on the focus object that the iOS plugin can query, and
give us a menu that has Copy instead of Paste.

Task-number: QTBUG-83811
Change-Id: Id8d18260002952fe7aa77aa5608cf7d0e440dacb
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/ios/qiostextresponder.mm

@@ -416,6 +416,17 @@
 
     if (unknownAction)
         return [super canPerformAction:action withSender:sender];
+
+    QObject *focusObject = QGuiApplication::focusObject();
+    if (focusObject && focusObject->property("qt_im_readonly").toBool()) {
+        // exceptional menu items for read-only views: do include Copy, do not include Paste etc.
+        if (action == @selector(cut:)
+                || action == @selector(paste:)
+                || action == @selector(delete:))
+            return NO;
+        if (action == @selector(copy:))
+            return YES;
+    }
     return (hasSelection && isEditAction) || (!hasSelection && isSelectAction);
 }