Add changes you made may not have been saved message to EditableMenuItem

[AndrewLester]

We should warn users before they exit the menu page so they don't lose unsaved changes. Use this code as a template for each EditableMenuItem:

<script>
function beforeUnload(e) {
    if (saved) return;

    e.returnValue = 'Unsaved';
    return e.returnValue;
}
</script>

<svelte:window on:beforeunload={beforeUnload}/>

[AndrewLester]

Unfortunately the current system only protects against refresh and URL bar navigation changes, NOT sveltekit routing changes. This would be a bug to report to sveltekit, but currently I don't see an easy way around this.