Add ability to change subcircuit buttons

[malmeloo]

This PR adds support for adding, removing and changing subcircuit buttons. An array of "Button" objects can be passed as an option to the constructor of a Circuit and will be applied to all subcircuit pop-ups. A button object currently looks like this:

{
    // unique button ID. Setting this to an existing ID overrides a previous button with the same ID.
    id: string,
    // (optional, default = false) Hides the button if `true`. Allows hiding existing buttons together with `id`.
    hidden: boolean,

    // The following two options are only relevant if the button is not hidden

    // The text to display inside the button
    buttonText: string,
    // function taking the circuit, model and paper objects of the subcircuit
    callback: ({circuit, model, paper}) => void
}

The current zoom in/out buttons are defined as defaults, so they will still be there if the new option is not specified.

Examples

Hiding existing buttons:

const buttons = [
    {
        id: "zoomIn",
        hidden: true
    },
    {
        id: "zoomOut",
        hidden: true
    }
]

const circuit = new digitaljs.Circuit(input, {subcircuitButtons: buttons});

Adding a new button:

const buttons = [
    {
        id: "magic",
        hidden: false,  // Optional!
        buttonText: "Do Magic",
        callback: ({circuit, model, paper}) => {
            console.log(model.get("label"));
        }
    }
]

const circuit = new digitaljs.Circuit(input, {subcircuitButtons: buttons});

Closes #99.