swkbd: Fix a bug where clicking Cancel hangs the game (#5294)

* swkbd: Fix a bug where clicking Cancel hangs the game

The text is validated in `Finalize`. If the validation fails, an error is returned and the applet is not actually finalized. This can result in hangs.

This is usually not a problem as the frontend is expected to validate the text passed to `Finalize`. However, when the user clicked on `Cancel`, the text is ignored and the frontend won't do any validation. Therefore, we should skip the validation here as well.

Also fixed a potential data race. All these functions should now be called on the same thread

* Address review comments

Renamed the fields
Remove close button

src/citra_qt/applets/swkbd.h

@@ -62,6 +62,9 @@ private:
 
     QWidget& parent;
 
+    std::string result_text;
+    int result_button;
+
     friend class QtKeyboardDialog;
     friend class QtKeyboardValidator;
 };