Fix SaveCurrentScreenshot

This fixes SaveCurrentScreenshot so it correctly saves the screenshot into the screenshot folder, it's no longer a stub

I was going to add the capture button so all games worked but tbh I only care for spongebob

games tested
Pokemon Z-A: https://files.raychu.xyz/u/1FaUGV.png
Pokemon Violet: https://files.raychu.xyz/u/6swfVS.png
Pokemon Violet: https://files.raychu.xyz/u/JaBBX2.png
Spongebob The Cosmic Shake: https://files.raychu.xyz/u/8z5X2e.png

src/Ryujinx/UI/Applet/AvaHostUIHandler.cs

@@ -260,5 +260,10 @@ namespace Ryujinx.Ava.UI.Applet
         {
             return new AvaloniaDynamicTextInputHandler(_parent);
         }
+        
+        public void TakeScreenshot()
+        {
+            _parent.ViewModel.AppHost.ScreenshotRequested = true;
+        }
     }
 }