More properties for MenuItem: `icon`

We should add a way to disable MenuItem and have an Icon

```slint
Menu {
   MenuItem { 
       title: @tr("Copy");
       icon: @image-url("icons/copy.png")
       enabled: root.has-selection;
       activated => { ... }
   } 
}
```

Update: `enabled` property was added in #8145 
This issue now tracks the `icon` property.