Allow icon to be customised through config

[johnpuddephatt]

This change keeps the icon as it is by default, but would optionally allow it to be customised in the config file.

My reason for suggesting this change is that currently the icon is the same as that used by the nova settings package.

[KasparRosin]

Would it work for you, if instead there was a function, where you could provide the new icon during registration?

    public function tools()
    {
        return [
            MenuBuilder::make()
                ->title('Custom Title'),
                ->icon('another-icon'),
        ];
    }

[KasparRosin]

Keeps the config file less bloated since the tool is initiated only once anyways.

[johnpuddephatt]

Yes this works.

I would submit another PR but I don't know how to do what you're suggesting!

[KasparRosin]

Merged via #161