Default Button Label appears in child classes of Button

[tejaede]

The default label in Button breaks backwards compatibility in classes that specialize Button. For example, take the following:

var Button = require("montage/ui/button.reel").Button;

exports.ActionButton = Button.specialize(/** @lends ActionButton.prototype */ {

With this commit "Button" is shoehorned into the element as seen below.

A quick fix on our end is to define empty label and defaultLabel properties in the Child class

var Button = require("montage/ui/button.reel").Button;

exports.ActionButton = Button.specialize(/** @lends ActionButton.prototype */ {

    defaultLabel: {
        value: ""
    },

    label: {
        value: ""
    },

However, the label is a breaking change if it is released and changes are not made in the implementing application.

For context, the use case for specializing button is two-fold:

1. Custom Styling
2. Act as link if a url is provided

[thibaultzanini]

@marchant FYI