Add support for system notifications when game and tab are unfocused

[danthedaniel]

Fixes: #118

Also removed a superfluous call to requestAnimationFrame()

[danthedaniel]

@creesch I could add in a new message type from the client to the server to enable the notification preferences when the user allows notification in the browser.

async function enableNotificationsOnClick() {
    if (await notificationManager.enableNotifications()) {
        console.log('Notifications enabled');
        sendWebsocketMessage('set_preferences', { systemNotifications: true });
    } else {
        console.log('Notifications not enabled');
    }

    document.body.removeEventListener('click', enableNotificationsOnClick);
}

Otherwise users may get confused when they accept the notifications in the browser but don't see them. Or I suppose that might not be an issue if we default to true. It will still be "opt-in".

We could also not have any server-side configuration and rely on the browser's preference configuration.

[creesch]

I completely forgot about this PR. We talked about it a little bit in chat. If I remember correctly what we discussed was possibly:

• Have a settings icon somewhere on the webpage
• Icon opens a settings window.
• Settings window has one setting for now "enable notifications in this browser".
• User clicks that, gets the browser notification permission screen and needs to also allow it there.
• For the web interface we then simply check on receiving messages if permission has been given.

Having said that, the current way you have implemented it does work nicely. Though to test it on a solo server I had to think for a second how to test it with both the game and chat not in focus. Opening up a second chat in a different browser fixed that :)